U.S. patent application number 15/027499 was filed with the patent office on 2016-09-08 for electrode material for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery electrode and non-aqueous electrolyte secondary battery using the same.
The applicant listed for this patent is NISSAN MOTOR CO., LTD.. Invention is credited to Hiroshi Akama, Hideaki Horie, Kenichi Kawakita, Yuki Kusachi, Yusuke Mizuno, Yuta Murakami, Yasuhiko Ohsawa, Yasuhiro Shindo, Yasuhiro Tsudo.
Application Number | 20160260966 15/027499 |
Document ID | / |
Family ID | 52813017 |
Filed Date | 2016-09-08 |
United States Patent
Application |
20160260966 |
Kind Code |
A1 |
Ohsawa; Yasuhiko ; et
al. |
September 8, 2016 |
Electrode material for non-aqueous electrolyte secondary battery,
and non-aqueous electrolyte secondary battery electrode and
non-aqueous electrolyte secondary battery using the same
Abstract
A core-shell-type electrode material is used as an electrode
active material layer of a non-aqueous electrolyte secondary
battery, the core-shell-type electrode material having a core part
in which at least a part of a surface of an electrode active
material is coated with a first conductive material and a shell
part in which a second conductive material is contained in a base
material formed by a gel-forming polymer having a tensile
elongation at break of 10% or more in a gel state.
Inventors: |
Ohsawa; Yasuhiko; (Kanagawa,
JP) ; Horie; Hideaki; (Kanagawa, JP) ; Akama;
Hiroshi; (Kanagawa, JP) ; Kusachi; Yuki;
(Kanagawa, JP) ; Murakami; Yuta; (Kyoto, JP)
; Kawakita; Kenichi; (Kyoto, JP) ; Mizuno;
Yusuke; (Kyoto, JP) ; Tsudo; Yasuhiro; (Kyoto,
JP) ; Shindo; Yasuhiro; (Kyoto, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSAN MOTOR CO., LTD. |
Yokohama-shi, Kanagawa |
|
JP |
|
|
Family ID: |
52813017 |
Appl. No.: |
15/027499 |
Filed: |
October 3, 2014 |
PCT Filed: |
October 3, 2014 |
PCT NO: |
PCT/JP2014/076597 |
371 Date: |
April 6, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 4/625 20130101;
H01M 4/66 20130101; H01M 4/485 20130101; H01M 10/0525 20130101;
H01M 2220/20 20130101; H01M 4/624 20130101; H01M 4/366 20130101;
H01M 4/139 20130101; H01M 4/131 20130101; Y02P 70/50 20151101; H01M
4/622 20130101; Y02T 10/70 20130101; Y02E 60/10 20130101 |
International
Class: |
H01M 4/36 20060101
H01M004/36; H01M 4/485 20060101 H01M004/485; H01M 10/0525 20060101
H01M010/0525; H01M 4/131 20060101 H01M004/131; H01M 4/62 20060101
H01M004/62; H01M 4/66 20060101 H01M004/66 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 7, 2013 |
JP |
2013-210562 |
Claims
1. An electrode material for a non-aqueous electrolyte secondary
battery used for a non-aqueous electrolyte secondary battery using
a liquid electrolyte, a gel polymer electrolyte, or an ionic liquid
electrolyte, the electrode material comprising: a core part in
which at least a part of a surface of an electrode active material
is coated with a first conductive material; and a shell part in
which a second conductive material is contained in a base material
formed by a gel-forming polymer having a tensile elongation at
break of 10% or more in a gel state.
2. The electrode material for a non-aqueous electrolyte secondary
battery according to claim 1, wherein the gel-forming polymer is a
polyurethane resin.
3. The electrode material for a non-aqueous electrolyte secondary
battery according to claim 2, wherein the polyurethane resin is the
one obtained by reaction of polyethylene glycol and an isocyanate
compound.
4. The electrode material for a non-aqueous electrolyte secondary
battery according to claim 1, wherein the electrode active material
is a metal oxide.
5. The electrode material for a non-aqueous electrolyte secondary
battery according to claim 1, wherein the first conductive material
is a carbon material.
6. A non-aqueous electrolyte secondary battery electrode obtained
by forming an electrode active material layer on a surface of a
current collector, the electrode active material layer comprising
an electrode material, the electrode material comprising: a core
part in which at least a part of a surface of an electrode active
material is coated with a first conductive material; and a shell
part in which a second conductive material is contained in a base
material formed by a gel-forming polymer having a tensile
elongation at break of 10% or more in a gel state.
7. The non-aqueous electrolyte secondary battery electrode
according to claim 6, wherein the electrode active material layer
comprises a binder, and the binder is an aqueous binder.
8. The non-aqueous electrolyte secondary battery electrode
according to claim 6, wherein the current collector comprises a
conductive resin layer formed by a resin having conductivity.
9. A non-aqueous electrolyte secondary battery comprising a power
generating element, the power generating element comprising: the
electrode set forth in claim 6; another electrode having a polarity
different from that of the electrode; and an electrolyte layer
interposed between active material layers of these two
electrodes.
10. The non-aqueous electrolyte secondary battery according to
claim 9, wherein the electrolyte layer comprises a liquid
electrolyte, a polymer gel electrolyte, or an ionic liquid
electrolyte.
11. The non-aqueous electrolyte secondary battery according to
claim 9, wherein the battery is a bipolar type lithium ion
secondary battery.
12. A method for producing an electrode material for a non-aqueous
electrolyte secondary battery, the method comprising a coating step
of coating a core part, in which at least a part of a surface of an
electrode active material is coated with a first conductive
material, with a shell part in which a second conductive material
is contained in a base material formed by a gel-forming polymer
having a tensile elongation at break of 10% or more in a gel
state.
13. The producing method according to claim 12, further comprising:
a step of preparing a mixture containing the base material and the
second conductive material in advance before the coating step,
wherein the coating step comprises a step of coating the core part
with the mixture.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is based on Japanese Patent
Application No. 2013-210562 filed on Oct. 7, 2013, the entire
content of which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to an electrode material for a
non-aqueous electrolyte secondary battery, and an electrode for a
non-aqueous electrolyte secondary battery and a non-aqueous
electrolyte secondary battery using the same.
BACKGROUND
[0003] Currently, a non-aqueous electrolyte secondary battery
including a lithium ion secondary battery, which is used for a
mobile device such as a mobile phone, is available as a commercial
product. Further, in recent years, it is desired to reduce the
amount of carbon dioxide in order to cope with the global warming.
As such, a non-aqueous electrolyte secondary battery having small
environmental burden has been used not only for a mobile device or
the like but also for a power source device of an electric vehicle
such as a hybrid vehicle (HEV), an electric vehicle (EV), or a fuel
cell vehicle.
[0004] The non-aqueous electrolyte secondary battery generally has
a configuration in which a positive electrode having a positive
electrode active material or the like applied on a current
collector and a negative electrode having a negative electrode
active material or the like applied on a current collector are
connected to each other via an electrolyte layer in which a
non-aqueous electrolyte solution or a non-aqueous electrolyte gel
is retained within a separator. Charging and discharging reactions
of a battery occur by absorption and desorption of ions such as
lithium ions on electrode active materials.
[0005] Herein, in an active material layer containing an electrode
active material, in general, a binder (adhesive) is contained, and
electrode active materials are bound to each other and come into
close contact with the current collector. Further, a conductive aid
is contained as necessary, and thus the conductivity of the active
material layer is secured. For example, JP 4-342966 A discloses a
technique that, when a negative electrode body formed by having
lithium or an alkali metal containing lithium as a main component
carried therein is used in a carbonaceous material that is a
calcined body of an organic compound, carboxymethyl cellulose and
styrene-butadiene rubber are concurrently used as a binder
(adhesive) of the negative electrode body. Further, according to
the disclosure of JP 4-342966 A, with such a configuration, it is
possible to prevent the negative electrode body from coming off or
prevent an internal short from occurring, and thus cycle
characteristics in charge and discharge can be improved.
SUMMARY
[0006] However, as a result of the investigation of the present
inventors, it is found that, in the related art in which a
component contained in an active material layer is bound by use of
a binder as in JP 4-342966 A, the internal resistance of a battery
cannot be sufficiently decreased (the internal resistance
increases) in some cases. If the internal resistance of the battery
can be lowered, a high power density battery with excellent rate
characteristics may be provided. For this reason, particularly in
an in-vehicle battery in which it is assumed that charge and
discharge are performed at a high current (high rate), a decrease
(suppressing an increase) in the internal resistance of the battery
is an urgent issue.
[0007] Under the circumstances, an object of the present invention
is to provide a means that can minimize an increase in the internal
resistance of a non-aqueous electrolyte secondary battery.
[0008] In order to achieve the object described above, according to
an embodiment of the present invention, there is provided an
electrode material for a non-aqueous electrolyte secondary battery,
having a core part in which at least a part of a surface of an
electrode active material is coated with a first conductive
material and a shell part in which a second conductive material is
contained in a base material formed by a gel-forming polymer having
a tensile elongation at break of 10% or more in a gel state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a cross-sectional view schematically illustrating
a bipolar type secondary battery according to an embodiment of the
present invention;
[0010] FIG. 2 is a cross-sectional view schematically illustrating
an embodiment of a core-shell-type electrode material;
[0011] FIG. 3 is a perspective view illustrating the appearance of
a flat lithium ion secondary battery as a representative embodiment
of a secondary battery;
[0012] FIG. 4 is a scanning electron microscope (SEM) photograph
(magnification of 5000) of a core-shell-type electrode material
(positive electrode material) obtained in Production Example 2 to
be described later;
[0013] FIG. 5 is a scanning electron microscope (SEM) photograph
(magnification of 5000) of a core-shell-type electrode material
(positive electrode material) obtained in Production Example 3 to
be described later;
[0014] FIG. 6 is a scanning electron microscope (SEM) photograph
(magnification of 5000) of a core-shell-type electrode material
(positive electrode material) obtained in Production Example 4 to
be described later; and
[0015] FIG. 7 is a scanning electron microscope (SEM) photograph
(magnification of 5000) of a core-shell-type electrode material
(positive electrode material) obtained in Production Example 5 to
be described later.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0016] According to an embodiment of the present invention, there
is provided an electrode material for a non-aqueous electrolyte
secondary battery, having a core part in which at least a part of a
surface of an electrode active material is coated with a first
conductive material and a shell part in which a second conductive
material is contained in a base material formed by a gel-forming
polymer having a tensile elongation at break of 10% or more in a
gel state.
[0017] According to the electrode material for a non-aqueous
electrolyte secondary battery having the above-described
configuration, the shell part containing a gel-forming polymer is
present on the surface of the core part containing an electrode
active material, and thus the conduction path of lithium ions from
the surface of the electrode active material is secured. In
addition, the electrode active material is coated with the
conductive material and the conductive material is included in the
gel-forming polymer, and thus the conduction path of electrons from
the surface of the electrode active material is also secured. As a
result, an increase in the internal resistance of the non-aqueous
electrolyte secondary battery can be minimized.
[0018] The present inventors conducted intensive studies in view of
the problems as described above (providing a means that can
minimize an increase in the internal resistance of a non-aqueous
electrolyte secondary battery). In the process, the present
inventors considered that an increase in the internal resistance of
a battery in the related art in which an active material layer
contains a binder is caused by the following mechanism. That is,
since the surface of the electrode active material is coated with
an insulating thin film derived from a binder, it is difficult to
sufficiently secure an electron conduction network from an
electrode active material to a current collector and an ion
conduction network toward a counter electrode. The present
inventors set a hypothesis that an increase in the internal
resistance of the battery occurs as a result. Also, this phenomenon
was considered to be the same as in a case where only carboxymethyl
cellulose is used as a binder (adhesive).
[0019] Regarding the hypothesis described above, the present
inventors, first, employed a configuration in which at least a part
of the surface of the electrode active material is coated with a
conductive material in order to facilitate the transfer of
electrons in the electrode active material and secure the electron
conduction network from the electrode active material to the
current collector. However, the present inventors found a problem
that even when the configuration in which at least a part of the
surface of the electrode active material is coated with a
conductive material is employed, it is difficult to sufficiently
suppress an increase of the internal resistance of the battery.
[0020] In addition, the present inventors tried disposing a shell
part formed by a polymer which forms a gel by swelling of a liquid
electrolyte (gel-forming polymer) and a conductive material on the
surface of the core part formed by an electrode active material,
based on a technical idea considerably different from the related
art. Then, it has been confirmed that both the conduction paths of
lithium ions and electrons on the surface of the electrode active
material are secured and a certain effect is obtained for decrease
in the internal resistance of the battery by such a
countermeasure.
[0021] However, it is also found that, when the shell part formed
by the gel-forming polymer and the conductive material is simply
disposed around the core part, the effect of lowering the internal
resistance is limited, and in some cases, the sufficient effect of
lowering the internal resistance cannot be obtained. Further, the
present inventors further examined the cause thereof, and found
that, even when the shell part is formed, the breakage of the shell
part is caused by expansion and shrinkage of the electrode active
material accompanying charge and discharge of the battery, and as a
result, the sufficient effect of lowering the internal resistance
cannot be achieved in some cases. Based on these findings, the
present inventors tried using a material having a certain level of
flexibility as a base material (gel-forming polymer) constituting
the shell part. Specifically, they confirmed that it is also
possible to follow expansion and shrinkage of the electrode active
material so that the shell part is less likely to be broken and the
sufficient effect of lowering the internal resistance can be
achieved when a gel-forming polymer having a tensile elongation at
break of 10% or more in a gel state is used as the gel-forming
polymer. Thus, the present invention is completed.
[0022] Hereinafter, while referring to the drawings, a description
will be made of a preferred embodiment according to the present
invention, but the technical scope of the present invention should
be determined based on the scope of claims, and is not limited only
to the following embodiments. Incidentally, the same reference
numerals are assigned to the same elements in the description of
the drawings, and duplicate descriptions are omitted. In addition,
the scale of the drawings includes some exaggeration for
descriptive reasons, and may thus be different from the actual
dimension.
[0023] In the present specification, in some cases, a bipolar type
lithium ion secondary battery is simply referred to as a "bipolar
type secondary battery" and a bipolar type lithium ion secondary
battery electrode is simply referred to as a "bipolar type
electrode."
[0024] FIG. 1 is a cross-sectional view schematically illustrating
a bipolar type secondary battery according to an embodiment of the
present invention. A bipolar type secondary battery 10 illustrated
in FIG. 1 has a configuration in which a power generating element
21 with a substantially rectangular shape, in which a charge and
discharge reaction actually occurs, is sealed in the inside of a
laminate film 29, which is a battery outer casing material.
[0025] As illustrated in FIG. 1, the power generating element 21 of
the bipolar type secondary battery 10 according to this embodiment
includes plural bipolar type electrodes 23 in which a positive
electrode active material layer 13 electrically connected to one
surface of a current collector 11 and a negative electrode active
material layer 15 electrically connected to the other surface of
the current collector 11 are formed. The respective bipolar type
electrodes 23 are stacked on top of each other via electrolyte
layers 17 to form the power generating element 21. Incidentally,
the electrolyte layers 17 each have a configuration in which an
electrolyte is held in the middle portion in the plane direction of
a separator serving as a base material. At this time, the bipolar
type electrodes 23 and the electrolyte layers 17 are alternately
stacked in such a manner that the positive electrode active
material layer 13 of one bipolar type electrode 23 faces the
negative electrode active material layer 15 of another bipolar type
electrode 23 adjacent to the one bipolar type electrode 23 via the
electrolyte layer 17. That is, the electrolyte layer 17 is disposed
to be interposed between the positive electrode active material
layer 13 of one bipolar type electrode 23 and the negative
electrode active material layer 15 of another bipolar type
electrode 23 adjacent to the one bipolar type electrode 23.
[0026] The positive electrode active material layer 13, the
electrolyte layer 17, and the negative electrode active material
layer 15, which are adjacent to each other, constitute a single
battery layer 19. Therefore, it can also be said that the bipolar
type secondary battery 10 has a configuration in which the single
battery layers 19 are stacked on top of each other. In addition, a
sealing portion (insulating layer) 31 is provided on the periphery
of each of the single battery layer 19. By this structure, liquid
junction caused by leakage of an electrolyte solution from the
electrolyte layer 17 is prevented and the contact between the
current collectors 11 adjacent to each other inside the battery or
occurrence of a short circuit caused by slight unevenness at the
end portion of the single battery layer 19 in the power generating
element 21 is prevented. Incidentally, the positive electrode
active material layer 13 is formed on only one surface in an
outermost layer positive electrode current collector 11a located on
the outermost layer of the power generating element 21. Further,
the negative electrode active material layer 15 is formed on only
one surface in an outermost layer negative electrode current
collector 11b located on the outermost layer of the power
generating element 21.
[0027] Further, in the bipolar type secondary battery 10
illustrated in FIG. 1, a positive electrode current collecting
plate 25 is disposed to be adjacent to the outermost layer positive
electrode current collector 11a, and extends to be exposed on the
outside of the laminate film 29, which is a battery outer casing
material. Meanwhile, a negative electrode current collecting plate
27 is disposed to be adjacent to the outermost layer negative
electrode current collector 11b and similarly extends to be exposed
on the outside of the laminate film 29.
[0028] Incidentally, the number of the single battery layer 19
stacked is adjusted depending on a desired voltage. Further, the
number of the single battery layer 19 stacked in the bipolar type
secondary battery 10 may be minimized as long as sufficient output
can be ensured even when the thickness of the battery is made
thinner as much as possible. In order to prevent external damage at
the time of operation and avoid environmental worsening, the
bipolar type secondary battery 10 may also have a structure in
which the power generating element 21 is sealed in the laminate
film 29, which is a battery outer casing material under reduced
pressure, and the positive electrode current collecting plate 25
and the negative electrode current collecting plate 27 are exposed
on the outside of the laminate film 29. Incidentally, herein, the
embodiment of the present invention has been described by using a
bipolar type secondary battery as an example, but the type of a
non-aqueous electrolyte battery to which the present invention can
be applied is not particularly limited. The present invention can
be applied to an arbitrary non-aqueous electrolyte secondary
battery of the related art, such as a so-called parallel laminate
type battery, in which single battery layers are connected in
parallel in a power generating element.
[0029] Hereinafter, main constituent elements of the bipolar type
secondary battery of this embodiment will be described.
[0030] <<Current Collector>>: A current collector
serves as a medium for transferring electrons from one side coming
into contact with the positive electrode active material layer to
the other side coming into contact with the negative electrode
active material layer. The material for forming the current
collector is not particularly limited, but, for example, a metal or
a resin having conductivity may be used.
[0031] Specific examples of the metal include aluminum, nickel,
iron, stainless steel, titanium, and copper. In addition to them, a
clad material of nickel and aluminum, a clad material of copper and
aluminum, or a plating material of a combination of those metals
may be preferably used. Furthermore, a foil obtained by coating a
metal surface with aluminum may be used. Among them, from the
viewpoint of electron conductivity, potential for operating a
battery, adhesiveness of a negative electrode active material to a
current collector by sputtering, or the like, aluminum, stainless
steel, copper, and nickel are preferable.
[0032] Further, examples of the latter resin having conductivity
include a resin in which conductive filler is added to a conductive
polymer material or a non-conductive polymer material as necessary.
Examples of the conductive polymer material include polyaniline,
polypyrrole, polythiophene, polyacetylene, polyparaphenylene,
polyphenylene vinylene, and polyoxadiazole. Such a conductive
polymer material has an advantage in simplification of the
manufacturing process and lightness of a current collector since
the conductive polymer material has sufficient conductivity even if
a conductive filler is not added thereto.
[0033] Examples of the non-conductive polymer material include
polyethylene (PE; high-density polyethylene (HDPE), low-density
polyethylene (LDPE), or the like), polypropylene (PP), polyethylene
terephthalate (PET), polyether nitrile (PEN), polyimide (PI),
polyamide imide (PAI), polyamide (PA), polytetrafluoroethylene
(PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN),
polymethyl acrylate (PMA), polymethylmethacrylate (PMMA), polyvinyl
chloride (PVC), polyvinylidene fluoride (PVdF), or polystyrene
(PS). Such a non-conductive polymer material may have excellent
potential tolerance or solvent tolerance.
[0034] A conductive filler may be added to the conductive polymer
material or the non-conductive polymer material as necessary. In
particular, when a resin serving as a base material of a current
collector only includes a non-conductive polymer, the conductive
filler is essential to provide the resin with conductivity.
[0035] The conductive filler can be used without particular
limitation as long as it is a material having conductivity.
Examples of a material having excellent conductivity, potential
tolerance, or lithium ion insulation include metal and conductive
carbon. The metal is not particularly limited, but the metal
preferably includes at least one kind of metal selected from the
group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb,
or an alloy or metal oxide containing these metals. Further, the
conductive carbon is not particularly limited. The conductive
carbon preferably includes at least one material selected from the
group consisting of acetylene black, Vulcan, Black Pearls, carbon
nanofiber, Ketjen black, carbon nanotube, carbon nanohorn, and
carbon nanoballoon.
[0036] The amount of the conductive filler added is not
particularly limited as long as it can provide the current
collector with sufficient conductivity. In general, the amount is
about 5 to 35% by mass.
[0037] Incidentally, the current collector of this embodiment may
be a single-layered structure formed by a single material or a
stacked structure in which layers formed by these materials are
appropriately combined. From the viewpoint of lightness of the
current collector, it is preferable to contain at least a
conductive resin layer formed by a resin having conductivity. In
addition, from the viewpoint of interrupting the movement of
lithium ions between single battery layers, a metal layer may be
provided on a portion of the current collector.
[0038] <<Positive Electrode Active Material Layer>>:
The positive electrode active material layer 13 contains a positive
electrode active material. The bipolar type secondary battery 10
according to this embodiment has a feature in the containing form
of the positive electrode active material contained in the positive
electrode active material layer 13.
[0039] Specifically, in the bipolar type secondary battery 10
according to this embodiment, the positive electrode active
material is contained in the positive electrode active material
layer 13 in the form of a core-shell-type electrode material
(core-shell-type positive electrode material) as illustrated in
FIG. 2.
[0040] A core-shell-type positive electrode material 13A
illustrated in FIG. 2 is configured by a core part 13c in which at
least a part of the surface of a positive electrode active material
13a is coated with a first conductive material 13b and a shell part
13s with which the surface of the core part 13c is coated.
[0041] Further, this core part 13c has a configuration in which a
metal oxide (for example, LiCoO.sub.2) as the positive electrode
active material 13a is coated with a carbon material as the first
conductive material 13b. Incidentally, in FIG. 2, the entire
surface of the positive electrode active material 13a is coated
with the first conductive material 13b, but as detailed later, a
part of the surface of the positive electrode active material 13a
may be exposed and come into direct contact with a base material
13d (the details thereof will be described later).
[0042] Further, the shell part 13s has a configuration in which an
acetylene black 13e as the second conductive material is contained
in the base material 13d formed by a predetermined polyethylene
glycol-based polyurethane resin. Incidentally, in FIG. 2, the
entire surface of the core part 13c is coated with the shell part
13s, but a part of the surface of the core part 13c may be exposed.
In a preferred embodiment, 50% by area or more of the surface of
the core part 13c is preferably coated with the shell part 13s,
more preferably 60% by area or more thereof, further preferably 70%
by area or more thereof, particularly preferably 80% by area or
more thereof, and most preferably 90% by area or more thereof
(upper limit: 100% by area).
[0043] Further, the mass ratio of the shell part 13s to the core
part 13c is not particularly limited, but the shell part 13s is
preferably 2 to 30 parts by mass, and more preferably 5 to 15 parts
by mass relative to 100 parts by mass of the core part 13c.
[0044] Hereinafter, the details of the core part 13c and the shell
part 13s will be described by illustrating, as an example, a case
where the core-shell-type electrode material according to the
present invention is a positive electrode material. However, as
described later, the present invention can be applied as a negative
electrode material.
[0045] <<Core Part 13c>>: In this embodiment, the core
part 13c contains the positive electrode active material 13a and
the first conductive material 13b.
[0046] The surface of the positive electrode active material 13a
according to this embodiment is coated with (supports) the first
conductive material 13b.
[0047] Incidentally, in the present specification, "coat" or
"support" means that the first conductive material is chemically or
physically bound to at least a portion of the surface of the
electrode active material. Further, it can be confirmed that the
surface of the electrode active material is coated with the first
conductive material by observing the produced electrode active
material or an electrode active material extracted (separated) from
the electrode using a well-known means such as a scanning electron
microscope (SEM). That is, it can be confirmed by performing
observation of electrode active material in a state in which the
first conductive material adheres to the active material particle,
by means of an SEM image or the like.
[0048] Herein, the coating ratio (support ratio) of the positive
electrode active material 13a by the first conductive material 13b
is not particularly limited. In consideration of the effect of
improving battery characteristics and conductivity, the coating
ratio (support ratio) of the positive electrode active material 13a
by the first conductive material is preferably 20% by area or more,
more preferably 50% by area or more, and still more preferably 75%
by area or more (upper limit: 100% by area).
[0049] In the present specification, as the "coating ratio (support
ratio) of the active material by the conductive material," a value
measured and calculated by Auger electron spectroscopy is
employed.
[0050] Herein, the method of controlling the coating ratio (support
ratio) with respect to the positive electrode active material 13a
by the first conductive material 13b to the above-described
preferable range is not particularly limited. Specifically, a
method can be used in which the positive electrode active material
13a and the first conductive material 13b or a raw material of the
first conductive material 13b are mixed at an appropriate ratio,
the resultant mixture is then physically or chemically treated, and
the first conductive material 13b is chemically or physically bound
to (applied to) the surface of the positive electrode active
material 13a.
[0051] In the above-described method, the mixing ratio of the
positive electrode active material 13a and the first conductive
material 13b (or a raw material of the first conductive material
13b) is not particularly limited. Specifically, when the total
amount of the positive electrode active material 13a and the first
conductive material 13b (or a raw material of the first conductive
material 13b) is considered as 100 parts by weight, the first
conductive material 13b (or a raw material of the first conductive
material 13b) is mixed at a ratio of preferably 0.1 to 50 parts by
weight. More preferably, in the above-described method, the first
conductive material 13b (or a raw material of the first conductive
material 13b) is mixed with the positive electrode active material
13a at a ratio of 1 to 25 parts by weight. When such a mixing ratio
is used, the coating ratio (support ratio) with respect to the
positive electrode active material 13a by the first conductive
material 13b can be easily controlled to the above-described
preferable range.
[0052] Hereinafter, the positive electrode active material 13a and
the first conductive material 13b which form the core part 13c will
be described in detail, respectively.
[0053] <<Positive Electrode Active Material 13a>>: The
positive electrode active material 13a has a composition which
absorbs ions during discharge and desorbs ions during charge.
[0054] As the positive electrode active material 13a, a metal oxide
is preferably used. In general, from the viewpoint of battery
characteristics (capacity), the metal oxide is practicable as a
positive electrode active material. However, when a metal oxide is
used as an active material, the metal oxide does not have a high
compatibility with a gel-forming polymer constituting the shell
part, and thus the adhesive force may not be sufficiently obtained
in some cases. On the other hand, in this embodiment, since the
surface of the positive electrode active material is coated with
the first conductive material, the adhesion property of the
gel-forming polymer to the positive electrode active material can
be improved. Therefore, in terms of the fact that the effect
obtained by using the first conductive material is significantly
exhibited, a metal oxide is used as a positive electrode active
material, which is a preferred embodiment in the present
invention.
[0055] Preferred examples of a metal oxide which is used as a
positive electrode active material include LiMn.sub.2O.sub.4,
LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2, Li.sub.4Ti.sub.5O.sub.12,
Li(Ni--Mn--Co)O.sub.2, and lithium-transition metal composite
oxide, such as a compound in which a part of these transition
metals is replaced with another element, a lithium-transition metal
phosphate compound such as LiFePO.sub.4, and a lithium-transition
metal sulfate compound. In some cases, two or more kinds of the
positive electrode active material may be concurrently used. From
the viewpoint of capacity and output characteristics,
lithium-transition metal composite oxide and a lithium-transition
metal phosphate compound are preferably used as a positive
electrode active material. A composite oxide containing lithium and
nickel is more preferably used, and Li(Ni--Mn--Co)O.sub.2 and a
composite oxide in which part of these transition metals is
replaced with another element (hereinafter, simply referred to as
"NMC composite oxide") is further preferably used. The NMC
composite oxide has a layered crystal structure in which a lithium
atom layer and a transition metal (Mn, Ni, and Co are arranged with
regularity) atom layer are alternately stacked via an oxygen atom
layer, one Li atom is included per atom of transition metal M, and
extractable Li amount is twice the amount of spinel lithium
manganese oxide, that is, as the supply power is two times higher,
it can have high capacity.
[0056] As described above, the NMC composite oxide also includes a
composite oxide in which a part of transition metal element is
replaced with another metal element. In this case, examples of
another metal element include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr,
Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, and Zn. Ti, Zr, Nb, W,
P, Al, Mg, V, Ca, Sr, and Cr are preferable, Ti, Zr, P, Al, Mg, and
Cr are more preferable, and from the viewpoint of improving the
cycle characteristics, Ti, Zr, Al, Mg, and Cr are further
preferable.
[0057] By having a high theoretical discharge capacity, the NMC
composite oxide preferably has a composition represented by General
Formula (1): Li.sub.aNi.sub.bMn.sub.cCo.sub.dM.sub.xO.sub.2 (with
the proviso that, in the formula, a, b, c, d, and x satisfy
0.9.ltoreq.a.ltoreq.1.2, 0<b<1, 0<c.ltoreq.0.5,
0<d.ltoreq.0.5, 0.ltoreq.x.ltoreq.0.3, b+c+d=1. M represents at
least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca,
Sr, and Cr). Herein, a represents the atomic ratio of Li, b
represents the atomic ratio of Ni, c represents the atomic ratio of
Co, d represents the atomic ratio of Mn, and x represents the
atomic ratio of M. From the viewpoint of the cycle characteristics,
it is preferable that 0.4.ltoreq.b.ltoreq.0.6 in General Formula
(1). Incidentally, the composition of each element can be measured
by induction coupled plasma (ICP) spectroscopy.
[0058] In general, from the viewpoint of improving purity and
improving electron conductivity of a material, nickel (Ni), cobalt
(Co), and manganese (Mn) are known to contribute to capacity and
output characteristics. Ti or the like replaces a part of
transition metal in a crystal lattice.
[0059] As a more preferred embodiment, from the viewpoint of
improving a balance between capacity and lifetime characteristics,
it is preferable that b, c, and d in General Formula (1) be
0.49.ltoreq.b.ltoreq.0.51, 0.29.ltoreq.c.ltoreq.0.31, and
0.19.ltoreq.d.ltoreq.0.21. For example, as compared with
LiCoO.sub.2, LiMn.sub.2O.sub.4,
LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2, and the like that exhibit
actual performance in a general consumer use battery,
LiNi.sub.0.5Mn.sub.0.3Co.sub.0.2O.sub.2 has a large capacity per
unit weight, and thus has an advantage that a compact battery
having a high capacity can be produced since the energy density can
be improved. In addition, from the viewpoint of a cruising
distance, LiNi.sub.0.5Mn.sub.0.3Co.sub.0.2O.sub.2 is preferable.
Incidentally, in terms of having a larger capacity,
LiNi.sub.0.8Co.sub.0.1Al.sub.0.1O.sub.2 is more advantageous, but
may have a problem in lifetime characteristics. On the other hand,
LiNi.sub.0.5Mn.sub.0.3CO.sub.0.2O.sub.2 has lifetime
characteristics as excellent as
LiNi.sub.1/3Mn.sub.1/3Co.sub.1/3O.sub.2.
[0060] Incidentally, it is needless to say that a positive
electrode active material other than the aforementioned positive
electrode active material may be used. In addition, the average
particle diameter of the positive electrode active material
contained in the positive electrode active material layer (in the
case of a core-shell-type electrode material, the average particle
diameter of a portion of the core part excluding the first
conductive material) is not particularly limited, but from the
viewpoint of having higher output power, the average particle
diameter is preferably 1 to 100 .mu.m, and more preferably 1 to 20
.mu.m.
[0061] <<First Conductive Material 13b>>: Regarding the
first conductive material 13b with which the positive electrode
active material 13a is coated, any materials may be used as long as
at least a part of the surface of the positive electrode active
material 13a can be coated with them and have conductivity. That
is, it is sufficient that the first conductive material 13b can
form a conductive path between the core part 13c and the surface of
the electrode material (the outer surface of the shell part
13s).
[0062] Examples of the first conductive material 13b include a
carbon material, a conductive metal oxide, a metal, conductive
ceramic, and a conductive polymer.
[0063] Among these materials described above, the first conductive
material 13b is preferably a carbon material. The positive
electrode active material 13a can strongly hold the base material
formed by the gel-forming polymer via a carbon material by
compatibility between the gel-forming polymer constituting the
shell part and the carbon material. That is, the core part 13c and
the shell part 13s are strongly attached to each other. Therefore,
it is possible to provide an electrode material with a stable
structure. Further, even when the conductivity of the positive
electrode active material 13a itself is low, by coating positive
electrode active material 13a with the carbon material, the
conductivity of the core part 13c can be improved due to the
conductivity of the carbon material.
[0064] Hereinafter, the carbon material will be described in
detail.
[0065] <<Carbon Material>>: The carbon material as the
first conductive material 13b is not particularly limited, and any
carbon materials may be used as long as at least a part of the
surface of the positive electrode active material 13a can be coated
with them. For example, the carbon material may be the same carbon
material to be used as a conductive aid (an additive to be blended
for improving the conductivity of the electrode active material
layer).
[0066] Specific examples thereof include acetylene black, furnace
black, carbon black, channel black, and graphite. Among these, from
the viewpoint of maintaining the coating of the carbon material by
suppressing Li ion insertion/removal, the carbon material
preferably has low crystallinity, and acetylene black is more
preferably used.
[0067] Further, the carbon material to be used may be appropriately
changed depending on a method used when the active material is
coated with the carbon material. Therefore, depending on a coating
method of the carbon material to the active material, a carbon
material other than those described above may be used. For example,
in a case where coating is performed by a sintering method to be
detailed later, a water-soluble polymer such as polyvinyl alcohol
or sucrose is preferably used as a carbon source (that is, a raw
material of the carbon material) for coating the active material.
Of them, polyvinyl alcohol is preferable.
[0068] The shape of the carbon material (the shape thereof in a
state in which the active material is coated with the carbon
material) is also not particularly limited, a particulate form or a
fibrous form may be employed. From the viewpoint of ease of
coating, a particulate form is preferable, and from the viewpoint
of conductivity, a fibrous form is preferable. The size of the
carbon material is also not particularly limited. For example, when
the carbon material is in a particulate form, the average particle
diameter (secondary particle diameter) thereof is preferably 10 to
200 nm, and more preferably 20 to 150 nm. Further, when the carbon
material is in a fibrous form, the diameter thereof is preferably
20 to 500 nm, and more preferably 50 to 300 nm, and the length
thereof is preferably 5 to 20 .mu.m, and more preferably 8 to 15
.mu.m. With such a size, the surface of the active material is
easily coated with the carbon material. In addition, with such a
size, the surface of the active material is uniformly coated with
the carbon material.
[0069] <<Coating Method of Carbon Material>>: In
particular, the coating method of the positive electrode active
material 13a when the first conductive material 13b is a carbon
material will be described. Regarding the coating method, a
physical or chemical treatment method for chemically or physically
binding (coating) the carbon material to the surface of the
positive electrode active material 13a is not particularly limited,
and examples thereof include a method of embedding at least a part
of the carbon material in the positive electrode active material
13a by shearing and a method of chemically binding the positive
electrode active material 13a and the surface of the carbon
material to each other via functional groups thereof. More
specifically, a sintering (calcining) method, a mechanochemical
method (surface treatment by a hybridizer), a liquid phase method,
a vapor deposition (CVD) method, or the like can be used. Among
these, a sintering (calcining) method and a mechanochemical method
are preferably used.
[0070] The physical or chemical treatment condition for chemically
or physically binding (coating) the carbon material to the positive
electrode active material 13a is not particularly limited, but can
be appropriately selected by a method to be used.
[0071] Hereinafter, as a preferable means as a method of coating
the carbon material of this embodiment, two methods will be
described in detail.
[0072] <<Sintering (Calcining) Method>>: When the
sintering (calcining) method is used, specifically, preparation is
preferably performed through the following steps. That is, first,
an aqueous solution of a water-soluble polymer is prepared (step
1). Next, the positive electrode active material 13a is dispersed
in the water-soluble polymer aqueous solution prepared in step 1
described above (step 2). Then, water is evaporated from the
aqueous solution prepared in step 2 described above, and a solid
thus obtained is dried (step 3). Further, the solid obtained in
step 3 described above is calcined (step 4). In this way, through
steps 1 to 4, it is possible to obtain the positive electrode
active material 13a coated with a carbon material (that is, the
core part 13c).
[0073] The concentration of the water-soluble polymer in the
water-soluble polymer aqueous solution in step 1 described above is
not particularly limited, but is preferably 0.1 to 50 parts by
weight, preferably 1 to 30 parts by weight, and particularly
preferably 1.5 to 10 parts by weight relative to 100 parts by
weight of water. In addition, when the water-soluble polymer
aqueous solution is prepared, the aqueous solution may be prepared
while being heated. At this time, the temperature of the aqueous
solution is not particularly limited, and is preferably 40 to
98.degree. C., and particularly preferably 50 to 95.degree. C.
[0074] In step 2 described above, the positive electrode active
material 13a is preferably dispersed while the water-soluble
polymer aqueous solution is stirred. At this time, the amount of
the positive electrode active material 13a to be dispersed in the
water-soluble polymer aqueous solution is not particularly limited,
but is preferably 0.1 to 50 parts by weight, preferably 1 to 30
parts by weight, and particularly preferably 5 to 20 parts by
weight relative to 100 parts by weight of the water-soluble polymer
aqueous solution. Further, the weight ratio of the water-soluble
polymer used in step 1 described above to the positive electrode
active material 13a is not particularly limited, but is preferably
1:99 to 99:1, more preferably 5:85 to 85:5, and further preferably
8:92 to 92:8. Incidentally, when the positive electrode active
material 13a is dispersed in the water-soluble polymer aqueous
solution, the aqueous solution may be prepared while being heated.
At this time, the temperature of the aqueous solution is not
particularly limited, but is preferably 40 to 98.degree. C., and
particularly preferably 50 to 95.degree. C.
[0075] In step 3 described above, in order to efficiently evaporate
water from the water-soluble polymer aqueous solution which is
prepared in step 2 described above and in which the positive
electrode active material 13a is dispersed, it is preferable to
heat the aqueous solution while being stirred. At this time, the
heating temperature is not particularly limited as long as it is a
temperature at which water is evaporated. After almost the whole
amount of water is evaporated, it is preferable to further dry the
obtained solid. The drying method used at this time is not
particularly limited, and methods such as natural drying, a
reduced-pressure drying method, and an air-blow drying method can
be used. Further, the drying temperature is not particularly
limited, but is preferably 100 to 180.degree. C., and more
preferably 110 to 160.degree. C.
[0076] In step 4 described above, the solid (a raw material of the
sintered body) obtained in step 3 described above can be calcined
by using a well-known calcining (sintering) device such as an
electric furnace or a belt furnace. By performing calcination in
the present step 4, it is possible to coat at least a part of the
surface of the positive electrode active material 13a with the
carbon material. At this time, the calcining temperature is not
particularly limited, but is preferably 200 to 1000.degree. C.,
more preferably 300 to 800.degree. C., and particularly preferably
350 to 500.degree. C. Further, the calcining time (heating time) is
also not particularly limited, but is preferably 10 minutes to 5
hours, more preferably 20 minutes to 3 hours, and particularly
preferably 30 minutes to 1 hour.
[0077] The positive electrode active material 13a coated with the
carbon material obtained through steps 1 to 4 described above (that
is, the core part 13c) may be pulverized to have a desired particle
diameter, if necessary.
[0078] <<Mechanochemical Method>>: In the
mechanochemical method, the surface of the positive electrode
active material 13a is coated with the carbon material by using
well-known devices such as ACM Pulperizer, Inomizer, impeller mill,
turbo mill, hammer mill, fine mill, Zepros, and hybridizers. At
this time, the rotation speed (treatment rotation speed) is
preferably 1,000 to 20,000 rpm, and more preferably 3,000 to 18,000
rpm. In addition, the load power is preferably 200 to 800 W, and
more preferably 400 to 650 W. The treatment time is preferably 1 to
60 minutes, and more preferably 2 to 10 minutes. If such conditions
are used, the carbon material can be applied to (carried in) the
surface of the positive electrode active material 13a at the
above-described preferable coating ratio (support ratio). Further,
the carbon material can be uniformly applied to the surface of the
positive electrode active material 13a.
[0079] <<Shell Part 13s>>: In this embodiment, the
shell part 13s has a configuration in which the second conductive
material 13e (here, acetylene black) is included in the base
material 13d formed by a predetermined polyethylene glycol-based
polyurethane resin as described above. Incidentally, in a case
where an electrolyte contained in an electrolyte layer to be
described later contains an electrolyte solution (that is, the
electrolyte is a liquid electrolyte or a gel electrolyte), an
electrolyte solution derived from an electrolyte contained in an
electrolyte layer is typically infiltrated in the positive
electrode active material layer 13. For this reason, the base
material (gel-forming polymer) constituting the shell part 13s
absorbs the electrolyte solution to be swollen so that the base
material becomes a gel state.
[0080] The thickness of the shell part is not particularly limited,
but as a thickness of a state in which a gel is not formed, the
thickness thereof is preferably 0.01 to 5 .mu.m, and more
preferably 0.1 to 2 .mu.m. In addition, as a thickness after the
shell part is immersed in an electrolyte solution (1M LiPF.sub.6,
ethylene carbonate (EC)/diethyl carbonate (DEC)=3/7 (volume ratio))
at 50.degree. C. for 3 days, the thickness thereof is preferably
0.01 to 10 .mu.m, and more preferably 0.1 to 5 .mu.m.
[0081] The constitution material of the base material 13d is not
limited to the polyethylene glycol-based polyurethane resin, and
may be a gel-forming polymer having a tensile elongation at break
of 10% or more in a gel state. The term "tensile elongation at
break" is an index representing the flexibility of the gel-forming
polymer, which is a constitution material of the base material 13d,
and is a value obtained by a measurement method described in the
section of Examples to be described later. The value of the tensile
elongation at break of the gel-forming polymer may be 10% or more,
preferably 20% or more, more preferably 30% or more, particularly
preferably 40% or more, and most preferably 50% or more. From the
viewpoint of solving the problems of the present invention, a
larger value of the tensile elongation at break of the gel-forming
polymer is preferable.
[0082] As a technique of providing flexibility to a gel-forming
polymer serving as a constituent material of the base material 13d
so as to control the tensile elongation at break to a desired
value, a method of introducing a partial structure having
flexibility (for example, a long-chain alkyl group, a polyether
residue, an alkyl polycarbonate residue, or an alkyl polyester
residue) to the main chain of the gel-forming polymer is
exemplified. In addition, it is also possible to provide
flexibility to a gel-forming polymer so as to adjust the tensile
elongation at break by a technique of controlling the molecular
weight of the gel-forming polymer or controlling the molecular
weight between crosslinks. Particularly, the gel-forming polymer is
preferably a polyurethane resin. When the polyurethane resin is
used as a gel-forming polymer, which is a constituent material of
the base material 13d, first, there is an advantage that a shell
part having high flexibility (large tensile elongation at break) is
formed. Further, since urethane bonds may form a strong hydrogen
bond with each other, it is possible to form a gel-forming polymer
having a stable structure while being excellent in flexibility.
[0083] When the gel-forming polymer is a polyurethane resin, the
specific form thereof is not particularly limited, and reference
can be made to the already-known knowledge.
[0084] The polyurethane resin is configured by (1) a polyisocyanate
component and (2) a polyol component, and may be configured by
further using (3) an ionic group-introducing component, (4) an
ionic group-neutralizing agent component, and (5) a chain extending
agent component, as necessary.
[0085] Examples of (1) the polyisocyanate component include
diisocyanate compounds having two isocyanate groups in one
molecule, and polyisocyanate compounds having three or more
isocyanate groups in one molecule, and these may be used alone or
in combination of two or more kinds thereof.
[0086] Examples of the diisocyanate compounds include aromatic
diisocyanates such as 4,4'-diphenylmethane diisocyanate(MDI),
2,4'-diphenylmethane diisocyanate, toluene-2,4-diisocyanate,
toluene-2,6-diisocyanate, p-phenylenediisocyanate, xylylene
diisocyanate, 1,5-naphthalene diisocyanate,
3,3'-dimethyldiphenyl-4,4'-diisocyanate, dianisidine diisocyanate,
and tetramethylxylylene diisocyanate; alicyclic diisocyanates such
as 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate,
cis-1,4-cyclohexyl diisocyanate, trans-1,4-cyclohexyl diisocyanate,
and norbornene diisocyanate; and aliphatic diisocyanates such as
1,6-hexamethylene diisocyanate, 2,2,4 and/or
(2,4,4)-trimethylhexamethylene diisocyanate, and lysine
diisocyanate.
[0087] These diisocyanate compounds may be used in the form of a
modified body such as carbodiimide-modified, isocyanurate-modified,
or biuret-modified, and also may be used in the form of a block
isocyanate blocked with various blocking agents.
[0088] Examples of the polyisocyanate compound having three or more
isocyanate groups in one molecule include isocyanurate trimers,
biuret trimers, and trimethylolpropane adducts of the diisocyanate
provided above as examples; and trifunctional or more isocyanate
such as triphenylmethane triisocyanate,
1-methylbenzole-2,4,6-triisocyanate, or dimethyl triphenylmethane
tetraisocyanate. These isocyanate compounds may be used in the form
of a modified body such as carbodiimide-modified,
isocyanurate-modified, or biuret-modified, and also may be used in
the form of a block isocyanate blocked with various blocking
agents.
[0089] Examples of (2) the polyol component include diol compounds
having two hydroxyl groups in one molecule and polyol compounds
having three or more hydroxyl groups in one molecule, and these may
be used alone or in combination of two or more kinds thereof.
[0090] Examples of the diol compounds and polyol compounds having
three or more hydroxyl groups in one molecule include low molecular
weight polyols, polyether polyols, polyester polyols, polyester
polycarbonate polyols, crystalline or noncrystalline polycarbonate
polyols, polybutadiene polyol, and silicone polyol.
[0091] Examples of the low molecular weight polyols include
aliphatic diols such as ethylene glycol, 1,2-propanediol,
1,3-propanediol, 2-methyl-1,3-propanediol,
2-butyl-2-ethyl-1,3-propanediol, 1,4-butanediol, neopentyl glycol,
3-methyl-2,4-pentanediol, 2,4-pentanediol, 1,5-pentanediol,
3-methyl-1,5-pentanediol, 2-methyl-2,4-pentanediol,
2,4-diethyl-1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol,
3,5-heptanediol, 1,8-octanediol, 2-methyl-1,8-octanediol,
1,9-nonanediol, and 1,10-decanediol; alicyclic diols such as
cyclohexanedimethanol and cyclohexanediol; and trihydric or higher
polyols such as trimethylolethane, trimethylolpropane, hexitols,
pentitols, glycerol, polyglycerol, pentaerythritol,
dipentaerythritol, and tetramethylolpropane.
[0092] Examples of the polyether polyols include ethylene oxide
adducts such as diethylene glycol, triethylene glycol,
tetraethylene glycol, and polyethylene glycol; propylene oxide
adducts such as dipropylene glycol, tripropylene glycol,
tetrapropylene glycol, and polypropylene glycol; and ethylene oxide
and/or propylene oxide adducts of the low molecular weight polyols
described above, and polytetramethylene glycol.
[0093] The polyester polyols include a polyester polyol obtained by
direct esterification and/or transesterification of a polyol such
as the low molecular weight polyols provided above as examples with
a polycarboxylic acid or its ester-forming derivative such as
ester, anhydride, or halide and/or a lactone or a hydroxycarboxylic
acid obtained by ring-opening hydrolysis of the lactone in an
amount less than the stoichiometric amount of the polyol. Examples
of the polycarboxylic acid or its ester-forming derivative include
polycarboxylic acids, such as aliphatic dicarboxylic acids such as
oxalic acid, malonic acid, succinic acid, glutaric acid, adipic
acid, pimeric acid, suberic acid, azelaic acid, sebacic acid,
dodecanedioic acid, 2-methylsuccinic acid, 2-methyladipic acid,
3-methyladipic acid, 3-methylpentanedioic acid, 2-methyloctanedioic
acid, 3,8-dimethyldecanedioic acid, 3,7-dimethyldecanedioic acid,
hydrogenated dimer acid, and dimer acid; aromatic dicarboxylic
acids such as phthalic acid, terephthalic acid, isophthalic acid,
and naphthalenedicarboxylic acid; alicyclic dicarboxylic acids such
as cyclohexanedicarboxylic acid; tricarboxylic acids such as
trimellitic acid, trimesic acid, and trimer of castor oil fatty
acid; and tetracarboxylic acids such as pyromellitic acid. Examples
of the ester-forming derivative include acid anhydrides of these
polycarboxylic acids; halides such as chlorides and bromides of the
polycarboxylic acids; and lower aliphatic esters such as methyl,
ethyl, propyl, isopropyl, butyl, isobutyl, and amyl esters of the
polycarboxylic acids. In addition, examples of the lactones include
lactones such as .gamma.-caprolactone, .delta.-caprolactone,
.epsilon.-caprolactone, dimethyl-.epsilon.-caprolactone,
.delta.-valerolactone, .gamma.-valerolactone, and
.gamma.-butyrolactone.
[0094] Examples of (3) the ionic group-introducing component, which
is used as necessary, include substances capable of introducing an
anionic group and substances capable of introducing a cationic
group. Examples of the substances capable of introducing an anionic
group include polyols containing a carboxyl group, such as
dimethylolpropionic acid, dimethylolbutanoic acid,
dimethylolbutyric acid, and dimethylolvaleric acid, and polyols
containing a sulfonic acid group, such as 1,4-butanediol-2-sulfonic
acid, and examples of the substances capable of introducing a
cationic group include N,N-dialkylalkanolamines,
N-alkyl-N,N-dialkanolamines such as N-methyl-N,N-diethanolamine and
N-butyl-N,N-diethanolamine, and trialkanolamines.
[0095] As (4) the ionic group-neutralizing agent component, which
is used as necessary, examples of anionic group neutralizers
include tertiary amine compounds such as trialkylamines (such as
trimethylamine, triethylamine, and tributylamine),
N,N-dialkylalkanolamines (such as N,N-dimethylethanolamine,
N,N-dimethyl propanolamine, N,N-dipropylethanolamine, and
1-dimethylamino-2-methyl-2-propanol), N-alkyl-N,N-dialkanolamines,
and trialkanolamines (such as triethanolamine); and basic compounds
(such as ammonia, trimethylammonium hydroxide, sodium hydroxide,
potassium hydroxide, and lithium hydroxide), and examples of
cationic group neutralizers include organic carboxylic acids such
as formic acid, acetic acid, lactic acid, succinic acid, glutaric
acid, and citric acid; organosulfonic acids such as paratoluene
sulfonic acid and alkyl sulfonates; inorganic acids such as
hydrochloric acid, phosphoric acid, nitric acid, and sulfonic acid;
epoxy compounds such as epihalohydrin; and quaternizing agents such
as dialkyl sulfates and alkyl halides.
[0096] As (5) the chain extending agent component which is used as
necessary, one or two or more kinds of commonly known chain
extending agents can be used, and polyamine compounds, polyhydric
primary alcohol compounds, and the like are preferable, and
polyamine compounds are more preferable. Examples of the polyamine
compounds include low molecular weight diamines resulting from the
substitution of an alcoholic hydroxyl group in the low molecular
weight diols provided as examples with an amino group, such as
ethylenediamine and propylenediamine; polyetherdiamines such as
polyoxypropylenediamine and polyoxyethylenediamine; alicyclic
diamines such as menthenediamine, isophoronediamine,
norbornenediamine, bis(4-amino-3-methyldicyclohexyl)methane,
diaminodicyclohexylmethane, bis(aminomethyl)cyclohexane, and
3,9-bis(3-aminopropyl)2,4,8,10-tetraoxaspiro(5,5)undecane; aromatic
diamines such as m-xylenediamine, .alpha.-(m/p
aminophenyl)ethylamine, m-phenylenediamine, diaminodiphenylmethane,
diaminodiphenylsulfone, diaminodiethyldimethyldiphenylmethane,
diaminodiethyldiphenylmethane, dimethylthiotoluenediamine,
diethyltoluenediamine, and
.alpha.,.alpha.'-bis(4-aminophenyl)-p-diisopropylbenzene;
hydrazine; and dicarboxylic acid dihydrazide compounds, which are
compounds formed of hydrazine and dicarboxylic acids provided as
examples of the polycarboxylic acid to be used for the
above-described polyester polyols.
[0097] Among the respective components described above, as (1) the
polyisocyanate component, a diisocyanate compound is preferably
used, 4,4'-diphenylmethane diisocyanate (MDI), 2,4'-diphenylmethane
diisocyanate, 4,4'-dicyclohexylmethanediisocyanate,
1,4-cyclohexyldiisocyanate, toluene-2,4-diisocyanate,
1,6-hexamethylene diisocyanate, or the like is particularly
preferably used, and diphenylmethane-4,4'-diisocyanate (MDI) is
most preferably used. In addition, as (2) the polyol component, it
is preferable that ethylene oxide adducts, which are diol
compounds, be necessarily used, and it is particularly preferable
that polyethylene glycol be necessarily used. That is, the
polyurethane resin is preferably the one obtained by reaction of
polyethylene glycol and an isocyanate compound. Since polyethylene
glycol is excellent in lithium ion conductivity, with such a
configuration, the effect of lowering (inhibiting an increase in)
the internal resistance of the battery may be significantly
exhibited. Herein, the number average molecular weight of
polyethylene glycol as calculated from a hydroxyl value is not
particularly limited, but is preferably 2,500 to 15,000, more
preferably 3,000 to 13,000, and further preferably 3,500 to 10,000.
From the viewpoint of heat resistance, in addition to the essential
components described above, it is preferable to further use
ethylene glycol and/or glycerol as a polyol component. In
particular, when only ethylene glycol is concurrently used without
using glycerol, a gel obtained by swelling of a gel-forming polymer
becomes a physically cross-linked gel, and thus a solvent can be
dissolved at the time of production. Further, various producing
methods as described later can be used. On the other hand, when
glycerol is also concurrently used in addition to ethylene glycol,
the main chains of a polyurethane resin are chemically cross-linked
to each other. In this case, there is an advantage that the
swelling degree to an electrolyte solution can be arbitrarily
controlled by controlling the molecular weight between
crosslinks.
[0098] Incidentally, the synthesis method of a polyurethane resin
is not particularly limited, and reference can be made to the
already-known knowledge.
[0099] In the above description, as a preferred embodiment of the
invention according to this embodiment, a case where the
gel-forming polymer is a polyurethane resin has been described in
detail, but it is needless to say that the configuration of the
gel-forming polymer is not limited thereto. For example, a
copolymer of polyvinylidene fluoride and hexafluoropropylene
(PVdF-HFP), polyacrylonitrile (PAN), (meth)acrylic resin or the
like is used as a gel-forming polymer, similarly. Further, even in
a case where these resins are used, it is possible to adjust a
tensile elongation at break by providing flexibility to a polymer
by using a technique of controlling the molecular weight of the
gel-forming polymer, designing a molecular structure, or the like
as described above.
[0100] In the shell part 13s, the specific type or the containing
form of the second conductive material 13e in the base material is
not particularly limited, but it is sufficient that a conductive
path between the core part 13c and the surface of the electrode
material (the outer surface of the shell part 13s) can be formed.
Further, the first conductive material 13b and the second
conductive material 13e may be formed by using the same material,
but they are clearly distinguished in the core-shell-type positive
electrode material 13A. That is, a material with which the surface
of the positive electrode active material 13a is directly coated is
the first conductive material 13b, and a material which is
dispersed in the shell part 13s is the second conductive material
13e. In addition, these materials are also clearly distinguished in
the production process as described below. In this embodiment, by
coating the surface of the positive electrode active material 13a
with the first conductive material 13b, the core part 13c is
produced in advance, and then the shell part 13s containing the
second conductive material 13e is formed on the surface of the core
part 13c. In this way, the first conductive material 13b and the
second conductive material 13e are distinguished from each other in
view of addition timing in the production step of the
core-shell-type positive electrode material 13A.
[0101] Examples of the type of the second conductive material 13e
includes carbon black such as Ketjen black or acetylene black;
carbon materials such as graphite and carbon fiber (for example,
vapor-phase growth carbon fiber (VGCF)); various kinds of carbon
nanotube (CNT), and other conductive fibers. As for the containing
form of the conductive material 13e, in a case where the second
conductive material 13e is acetylene black or a material which may
have a filamentous structure such as carbon fiber, as illustrated
in FIG. 2, it is preferable that the surface of the electrode
active material 13a (the positive electrode active material in FIG.
2) or the first conductive material 13b constituting the core part
13c and the surface of the electrode material (the outer surface of
the shell part 13s) be electrically conducted via the second
conductive material 13e.
[0102] The ratio of the content of the base material 13d and the
second conductive material 13e contained in the shell part 13s is
also not particularly limited, but, for example, the content of the
second conductive material is preferably 10 to 400 parts by mass,
and more preferably 25 to 150 parts by mass relative to 100 parts
by mass of the base material.
[0103] When the content of the conductive material is 10 parts by
mass or more, it is possible to form a sufficient conductive path,
which contributes to a decrease (suppressing an increase) in the
internal resistance of the battery. On the other hand, when the
content of the conductive material is 400 parts by mass or less,
from the viewpoints of having stability of the shell layer, the
above content is preferable. Incidentally, a value of the ratio of
those contents thereof is calculated as an average value of values
obtained by measuring 50 or more of core-shell-type electrode
active materials.
[0104] As described above, according to the non-aqueous electrolyte
secondary battery related to this embodiment, the effect of
lowering (suppressing an increase in) the internal resistance of
the battery is obtained, because the surface of the core part 13c
is coated with the shell part 13s. The invention according to the
embodiment can be carried out by referring to the disclosure
regarding the inventions of the electrode material according the
aforementioned embodiment.
[0105] <<Method for Producing Core-Shell-Type Electrode
Material>>: The method for producing a core-shell-type
electrode material is not particularly limited, but any techniques
may be preferably used as long as the method includes a coating
step of coating a core part in which at least a part of a surface
of an electrode active material is coated with a first conductive
material with a shell part in which a second conductive material is
included in a base material formed by a gel-forming polymer having
a tensile elongation at break of 10% or more in a gel state. For
example, the following three techniques are exemplified.
[0106] (1) Poor Solvent Precipitation Method (Referring to Section
of Examples to be described Later). In this method, first, a
gel-forming polymer as a base material constituting the shell part
13s is dissolved in a good solvent (in a case where the gel-forming
polymer is the polyurethane resin described above, for example,
N,N-dimethylformamide (DMF)). Next, powder of an electrode active
material coated with the first conductive material is dispersed in
the solution, and a poor solvent of the gel-forming polymer (in a
case where the gel-forming polymer is the polyurethane resin
described above, for example, isopropanol (IPA)) is added to the
solution. In this method, the gel-forming polymer is precipitated
on the surface of the electrode active material and on the surface
of the first conductive material based on the amount of the poor
solvent added and the coating of the core part with the gel-forming
polymer is achieved. As necessary, the addition of the poor solvent
may be separately performed in plural times, or the poor solvent
can also be contained in the original solution. At this time, when
the second conductive material is dispersed in a poor solvent to be
added at any time of addition of the poor solvent, the second
conductive material can be contained in advance in the gel-forming
polymer to be precipitated, and thus a core-shell-type electrode
material as illustrated in FIG. 2 can be obtained. Incidentally,
the above-described operation may be repeated in such a manner that
the solid content is filtered by a technique such as filtration
under reduced pressure after a predetermined amount of the shell
part is formed, and then the filtered solid content is dissolved in
the above-described good solvent. At this time, the distribution of
the conductive material in the shell part 13s can also be
controlled to have a desired form by differentiating the amount
(concentration) of the second conductive material to be contained
in the gel-forming polymer to be precipitated.
[0107] (2) Sugar Coating Method (Simple Spray Drying Method). In
this method, first, a solution is prepared by dissolving the second
conductive material and a gel-forming polymer in a good solvent of
the gel-forming polymer. Next, the obtained solution is sprayed on
the surface of an electrode active material coated with the first
conductive material, and then, as necessary, drying treatment is
carried out under stirring. Thus, it is possible to obtain a
core-shell-type electrode material as illustrated in FIG. 2 by
means of the simple technique.
[0108] (3) Solid Grinding Method. In this method, a solution is
prepared by dissolving an electrode active material coated with the
first conductive material, the second conductive material, and a
gel-forming polymer in a good solvent of the gel-forming polymer.
Next, this solution is spread on, for example, a tray, and is dried
at a temperature of about 60 to 100.degree. C. so as to evaporate
the solvent. In this way, the obtained solid matter is ground to
have a desired particle diameter, and, as necessary, screening is
carried out. Even in such a technique, it is possible to obtain a
core-shell-type electrode material as illustrated in FIG. 2 by
means of the simple technique.
[0109] Among the above (1) to (3), from the viewpoint of the
completeness of coating, the method of (1) described above is
preferable. In addition, from the viewpoint of ease of the process,
the methods of (2) and (3) described above, which further include a
step of preparing a mixture containing the base material
(gel-forming polymer) and the conductive material in advance before
the coating step of the electrode active material, are
preferable.
[0110] Hereinbefore, the specific embodiment of the core-shell-type
positive electrode material contained in the positive electrode
active material layer 13, which is the characteristic configuration
in this embodiment, has been described, but the positive electrode
active material layer 13 may contain a positive electrode active
material other than the aforementioned core-shell-type positive
electrode material (for example, the same material as in the
related art). Further, in addition to the positive electrode active
material (including the core-shell-type positive electrode
material), the positive electrode active material layer 13 may
contain a binder, a conductive aid, an ion conductive polymer, a
lithium salt, or the like.
[0111] Examples of the binder include a solvent-based binder such
as polyvinylidene fluoride (PVdF), and an aqueous binder.
[0112] The electrode active material layer preferably contains at
least an aqueous binder. The aqueous binder has a high binding
property. Further, since water as a raw material is easily
available and also only water vapor is generated during drying,
there is an advantage that the investment on facilities of a
production line can be greatly reduced and an environmental load
can be reduced. In addition, when an aqueous binder is used as a
binder to be contained in the active material layer in the present
invention, water is used as a solvent for preparing an active
material slurry which is prepared at the time of coating of the
active material layer. However, in this case, even when a
core-shell-type electrode material is further added to the active
material slurry, the risk that a gel-forming material constituting
the electrode material is dissolved in water serving as a
preparation solvent is small. For this reason, there are also
advantages that it is possible to stably use the electrode material
and a gel-forming polymer, which may form a physically cross-linked
gel can be used in production of the electrode material.
[0113] The aqueous binder indicates a binder which has water as a
solvent or a dispersion medium, and specific examples thereof
include a thermoplastic resin, a polymer with rubber elasticity, a
water soluble polymer, and a mixture thereof. Herein, the binder
which has water as a dispersion medium includes all expressed as
latex or emulsion, and it indicates a polymer emulsified in water
or suspended in water. Examples thereof include a polymer latex
obtained by emulsion polymerization in a self-emulsifying
system.
[0114] Specific examples of the aqueous binder include a styrene
polymer (styrene-butadiene rubber, a styrene-vinyl acetate
copolymer, a styrene-acryl copolymer, or the like),
acrylonitrile-butadiene rubber, methyl methacrylate-butadiene
rubber, a (meth)acrylic polymer (polyethylacrylate,
polyethylmethacrylate, polypropylacrylate, polymethylmethacrylate
(methyl methacrylate rubber), polypropylmethacrylate,
polyisopropylacrylate, polyisopropylmethacrylate,
polybutylacrylate, polybutylmethacrylate, polyhexylacrylate,
polyhexylmethacrylate, polyethylhexylacrylate,
polyethylhexylmethacrylate, polylaurylacrylate,
polylaurylmethacrylate, or the like), polytetrafluoroethylene,
polyethylene, polypropylene, an ethylene-propylene copolymer,
polybutadiene, butyl rubber, fluororubber, polyethylene oxide,
polyepichlorohydrin, polyphosphagen, polyacrylonitrile,
polystyrene, an ethylene-propylene-diene copolymer,
polyvinylpyridine, chlorosulfonated polyethylene, a polyester
resin, a phenol resin, an epoxy resin; polyvinyl alcohol (the
average polymerization degree is preferably 200 to 4000, and more
preferably 1000 to 3000, and the saponification degree is
preferably 80% by mol or more, and more preferably 90% by mol or
more) and a modified product thereof (1 to 80% by mol saponified
product in a vinyl acetate unit of a copolymer with ethylene/vinyl
acetate=2/98 to 30/70 (molar ratio), 1 to 50% by mol partially
acetalized product of polyvinyl alcohol, or the like), starch and a
modified product thereof (oxidized starch, phosphoric acid
esterified starch, cationized starch, or the like), cellulose
derivatives (carboxymethyl cellulose, methyl cellulose,
hydroxypropyl cellulose, hydroxyethyl cellulose, and salts
thereof), polyvinylpyrrolidone, polyacrylic acid (salt), a
copolymer of (meth)acrylamide and/or (meth)acrylic acid salt [a
(meth)acrylamide polymer, a (meth)acrylamide-(meth)acrylic acid
salt copolymer, an alkyl (carbon atom number of 1 to 4)
(meth)acrylate-(meth)acrylic acid salt copolymer, or the like], a
styrene-maleic acid salt copolymer, a mannich modified product of
polyacrylamide, a formalin condensation type resin (a urea-formalin
resin, a melamin-formalin resin, or the like), a polyamidepolyamine
or dialkylamine-epichlorohydrin copolymer, polyethyleneimine,
casein, soybean protein, synthetic protein, and a water soluble
polymer such as galactomannan derivatives. These aqueous binders
may be used alone or in combination of two or more kinds
thereof.
[0115] From the viewpoint of a binding property, the aqueous binder
preferably contains at least one rubber-based binder selected from
the group consisting of styrene-butadiene rubber,
acrylonitrile-butadiene rubber, methyl methacrylate-butadiene
rubber, and methyl methacrylate rubber. Moreover, from the
viewpoint of having a good binding property, the aqueous binder
preferably contains styrene-butadiene rubber.
[0116] When styrene-butadiene rubber is used as an aqueous binder,
the aforementioned water soluble polymer is preferably used in
combination from the viewpoint of improving the coating property.
Examples of the water soluble polymer which is preferably used in
combination with styrene-butadiene rubber include polyvinyl alcohol
and a modified product thereof, starch and a modified product
thereof, cellulose derivatives (carboxymethyl cellulose, methyl
cellulose, hydroxyethyl cellulose, and salts thereof),
polyvinylpyrrolidone, polyacrylic acid (salt), and polyethylene
glycol. Among them, styrene-butadiene rubber and carboxymethyl
cellulose (salt) are preferably combined as a binder. The weight
content ratio of the styrene-butadiene rubber to the water soluble
polymer is not particularly limited, but the styrene-butadiene
rubber: the water soluble polymer is preferably 1:0.1 to 10, and
more preferably 1:0.5 to 2.
[0117] The conductive aid means an additive which is blended in
order to enhance the conductivity of the electrode active material
layer. Examples of the conductive aid include carbon materials such
as carbon black including Ketjen black, acetylene black, and the
like, graphite, and carbon fiber, which are similar to the
above-mentioned the second conductive material. When the active
material layer contains a conductive aid, an electron network in
the inside of the active material layer is effectively formed, and
it can contribute to improvement of the output characteristics of a
battery.
[0118] Examples of the ion conductive polymer include polyethylene
oxide (PEO)-based and polypropylene oxide (PPO)-based polymers.
[0119] Examples of the electrolyte salt (lithium salt) include
Li(C.sub.2F.sub.5SO.sub.2).sub.2N, LiPF.sub.6, LiBF.sub.4,
LiClO.sub.4, LiAsF.sub.6, and LiCF.sub.3SO.sub.3.
[0120] A blending ratio of the components that are contained in the
positive electrode active material layer 13 and the negative
electrode active material layer 15 to be described later is not
particularly limited. The blending ratio can be adjusted by
suitably referring to the already-known knowledge about a lithium
ion secondary battery. Also, the thickness of each active material
layer is not particularly limited, and reference can be made to the
already-known knowledge about a battery. For example, the thickness
of each active material layer is about 2 to 100 .mu.m.
[0121] <<Negative Electrode Active Material Layer>>:
The negative electrode active material layer 15 contains a negative
electrode active material. Further, the negative electrode active
material layer 15 may contain a binder, a conductive aid, an ion
conductive polymer, a lithium salt, or the like in addition to the
negative electrode active material. Since the details of the
negative electrode active material layer are basically the same as
the details described in the section of "Positive Electrode Active
Material Layer" except the type of the negative electrode active
material, description thereof will be omitted. In other words, in
the descriptions referring to FIG. 1 and FIG. 2, a case where the
positive electrode active material layer 13 contains a
core-shell-type electrode material (positive electrode material)
has been described as an example, but the present invention can
also be applied to the negative electrode. That is, the negative
electrode active material contained in the negative electrode
active material layer 15 may be a core-shell-type electrode
material (negative electrode material) according to the present
invention.
[0122] Examples of the negative electrode active material include a
carbon material such as graphite (black lead), soft carbon, or hard
carbon, a lithium-transition metal composite oxide (for example,
Li.sub.4Ti.sub.5O.sub.12), a metal material, and a lithium
alloy-based negative electrode material. In some cases, two or more
kinds of a negative electrode active material may be concurrently
used. Preferably, from the viewpoint of capacity and output
characteristics, a carbon material or a lithium-transition metal
composite oxide is used as a negative electrode active material.
Incidentally, it is needless to say that a negative electrode
active material other than those described above may also be
used.
[0123] Further, a base material (gel-forming polymer) constituting
the shell part in the core-shell-type electrode material according
to the present invention has a property of easily adhering to
particularly a carbon material. For this reason, in a case where
the core-shell-type electrode material of the present invention is
applied to a negative electrode, from the viewpoint of providing an
electrode material with a stable structure, it is preferable in the
present invention to use a carbon material as a negative electrode
active material. With such a configuration, the base material
(gel-forming polymer) easily adheres to the surface of the negative
electrode active material which is not coated with the first
conductive material, and thus an electrode material with a more
stable structure is provided.
[0124] The average particle diameter of the negative electrode
active material (in the case of a core-shell-type electrode
material, the average particle diameter of a portion of the core
part excluding the first conductive material) is not particularly
limited, but from the viewpoint of higher output power, is
preferably 1 to 100 .mu.m, and more preferably 1 to 20 .mu.m.
[0125] <<Electrolyte Layer>>: The electrolyte to be
used in the electrolyte layer 17 of this embodiment is not
particularly limited, and a liquid electrolyte, a gel polymer
electrolyte, or an ionic liquid electrolyte is used without
limitation.
[0126] The liquid electrolyte has a function as a lithium ion
carrier. The liquid electrolyte constituting an electrolyte
solution layer has the form in which lithium salt is dissolved in
an organic solvent. Examples of the organic solvent which can be
used include carbonates such as ethylene carbonate (EC), propylene
carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC),
and ethylmethyl carbonate. Further, as a lithium salt, a compound
which can be added to an active material layer of an electrode such
as Li(C.sub.2F.sub.5SO.sub.2).sub.2N, LiPF.sub.6, LiBF.sub.4,
LiClO.sub.4, LiAsF.sub.6, or LiCF.sub.3SO.sub.3 can be similarly
employed. The liquid electrolyte may further contain an additive in
addition to the components described above. Specific examples of
such a compound include vinylene carbonate, methylvinylene
carbonate, dimethylvinylene carbonate, phenylvinylene carbonate,
diphenylvinylene carbonate, ethylvinylene carbonate,
diethylvinylene carbonate, vinylethylene carbonate,
1,2-divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate,
1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene
carbonate, 1-ethyl-2-vinylethylene carbonate, vinylvinylene
carbonate, allylethylene carbonate, vinyloxymethylethylene
carbonate, allyloxymethylethylene carbonate, acryloxymethylethylene
carbonate, methacryloxymethylethylene carbonate, ethynylethylene
carbonate, propargylethylene carbonate, ethynyloxymethylethylene
carbonate, propargyloxyethylene carbonate, methylene ethylene
carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. Among
them, vinylene carbonate, methylvinylene carbonate, and
vinylethylene carbonate are preferable, and vinylene carbonate and
vinylethylene carbonate are more preferable. These cyclic carbonate
esters may be used alone or in combination of two or more kinds
thereof.
[0127] The gel polymer electrolyte has a configuration in which the
aforementioned liquid electrolyte is injected to a matrix polymer
(host polymer) consisting of an ion conductive polymer. Using the
gel polymer electrolyte as an electrolyte is excellent in that the
fluidity of an electrolyte disappears and ion conductivity between
layers is easily blocked. Examples of an ion conductive polymer
which is used as a matrix polymer (host polymer) include
polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene
glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride-hex
afluoropropylene (PVdF-HEP), poly(methyl methacrylate (PMMA), and
copolymers thereof.
[0128] The matrix polymer of a gel polymer electrolyte can exhibit
excellent mechanical strength by forming a cross-linked structure.
For forming a cross-linked structure, it is sufficient to perform a
polymerization treatment of a polymerizable polymer for forming a
polymer electrolyte (for example, PEO and PPO), such as thermal
polymerization, UV polymerization, radiation polymerization, and
electron beam polymerization, by using a suitable polymerization
initiator.
[0129] The ionic liquid electrolyte is obtained by dissolving a
lithium salt in an ionic liquid. Incidentally, the ionic liquid is
a salt composed of cation and anion alone, and represents a series
of compounds which is liquid at normal temperature.
[0130] Regarding the cationic component constituting an ionic
liquid, it is preferable to use at least one selected from the
group consisting of a substituted or unsubstituted imidazolium ion,
a substituted or unsubstituted pyridinium ion, a substituted or
unsubstituted pyrrolium ion, a substituted or unsubstituted
pyrazolium ion, a substituted or unsubstituted pyrrolinium ion, a
substituted or unsubstituted pyrrolidinium ion, a substituted or
unsubstituted piperidinium ion, a substituted or unsubstituted
triazinium ion, and a substituted or unsubstituted ammonium
ion.
[0131] Specific examples of the anionic component constituting an
ionic liquid include halide ion such as fluoride ion, chloride ion,
bromide ion, or iodide ion, nitric acid ion (NO.sub.3.sup.-),
tetrafluoroborate ion (BF.sub.4.sup.-), hexafluorophosphate ion
(PF.sub.6.sup.-), (FSO.sub.2).sub.2N.sup.-, AlCl.sub.3.sup.-,
lactic acid ion, acetate ion (CH.sub.3COO.sup.-), trifluoroacetate
ion (CF.sub.3COO.sup.-), methanesulfonate ion
(CH.sub.3SO.sub.3.sup.-), trifluoromethanesulfonate ion
(CF.sub.3SO.sub.3.sup.-), bis(trifluoromethanesulfonyl)imide ion
((CF.sub.3SO.sub.2).sub.2N.sup.-),
bis(pentafluoroethylsulfonyl)imide ion
((C.sub.2F.sub.5SO.sub.2).sub.2N.sup.-),
BF.sub.3C.sub.2F.sub.5.sup.-, tris(trifluoromethanesulfonyl)carbon
acid ion ((CF.sub.3SO.sub.2).sub.3C.sup.-), perchlorate ion
(ClO.sub.4.sup.-), dicyanamide ion ((CN).sub.2N.sup.-), organic
sulfuric acid ion, organic sulfonic acid ion, R.sup.1COO.sup.-,
HOOCR.sup.1COO.sup.-, .sup.-OOCR.sup.1COO.sup.-, and
NH.sub.2CHR.sup.1COO.sup.- (in this case, R.sup.1 is a substituent
and represents an aliphatic hydrocarbon group, an alicyclic
hydrocarbon group, an aromatic hydrocarbon group, an ether group,
an ester group, or an acyl group, and the substituent may include a
fluorine atom).
[0132] Preferred examples of the ionic liquid include
1-methyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,
and N-methyl-N-propylpyrrolidinium
bis(trifluoromethanesulfonyl)imide. These ionic liquids may be used
alone or in combination of two or more kinds thereof.
[0133] The lithium salt which is used in the ionic liquid
electrolyte is the same lithium salt which is used in the liquid
electrolyte described above. Incidentally, the concentration of the
lithium salt is preferably 0.1 to 2.0 M, and more preferably 0.8 to
1.2 M.
[0134] An additive as described below may be added to the ionic
liquid. When an additive is contained, charge/discharge
characteristics and cycle characteristics may be further improved
at a high rate. Specific examples of the additive include vinylene
carbonate, ethylene carbonate, propylene carbonate,
.gamma.-butylolactone, .gamma.-valerolactone, methyl diglyme,
sulfolane, trimethylphosphate, triethylphosphate, methoxymethyl
ethyl carbonate, and fluorinated ethylene carbonate. These may be
used alone or in combination of two or more kinds thereof. The use
amount when an additive is used is preferably 0.5 to 10% by mass,
and more preferably 0.5 to 5% by mass with respect to the ionic
liquid.
[0135] In the bipolar type secondary battery of this embodiment, a
separator may be used in an electrolyte layer. The separator has a
function of holding an electrolyte so as to secure the lithium ion
conductivity between a positive electrode and a negative electrode
and a function of serving as a partition wall between a positive
electrode and a negative electrode. In particular, in a case where
a liquid electrolyte or an ionic liquid electrolyte is used as an
electrolyte, it is preferable to use a separator.
[0136] Examples of a separator shape include a porous sheet
separator or a non-woven separator composed of a polymer or a fiber
which absorbs and maintains the electrolyte.
[0137] As a porous sheet separator composed of a polymer or a
fiber, a microporous (microporous membrane) separator can be used,
for example. Specific examples of the porous sheet composed of a
polymer or a fiber include a microporous (microporous membrane)
separator which is composed of polyolefin such as polyethylene (PE)
and polypropylene (PP); a laminate in which a plurality of them are
laminated (for example, a laminate with three-layer structure of
PP/PE/PP), and a hydrocarbon based resin such as polyimide, aramid,
or polyfluorovinylydene-hexafluoropropylene (PVdF-HFP), or glass
fiber.
[0138] The thickness of the microporous (microporous membrane)
separator cannot be uniformly defined as it varies depending on use
of application. For example, for an application in a secondary
battery for operating a motor of an electric vehicle (EV), a hybrid
electric vehicle (HEV), and a fuel cell vehicle (FCV), it is
preferably 4 to 60 .mu.m as a single layer or a multilayer. The
fine pore diameter of the microporous (microporous membrane)
separator is preferably 1 .mu.m or less at most (in general, the
pore diameter is about several tens of nanometers).
[0139] As a non-woven separator, conventionally known ones such as
cotton, rayon, acetate, nylon, and polyester; polyolefin such as PP
and PE; polyimide and aramid are used either singly or as a
mixture. Further, the bulk density of the non-woven separator is
not particularly limited as long as sufficient battery
characteristics are obtained with an impregnated polymer gel
electrolyte. Furthermore, it is sufficient that the thickness of
the non-woven separator is the same as that of an electrolyte
layer, and the thickness thereof is preferably 5 to 200 .mu.m, and
particularly preferably 10 to 100 .mu.m.
[0140] Further, as a separator, a separator in which a heat
resistant insulating layer is laminated on a porous substrate (a
separator having a heat resistant insulating layer) is preferable.
The heat resistant insulating layer is a ceramic layer containing
inorganic particles and a binder. As for the separator having a
heat resistant insulating layer, those having high heat resistance,
that is, a melting point or a heat softening point of 150.degree.
C. or higher, preferably 200.degree. C. or higher, are used. By
having a heat resistant insulating layer, internal stress in a
separator, which increases under temperature increase, is
alleviated so that the effect of inhibiting thermal shrinkage can
be obtained. As a result, an occurrence of a short between
electrodes of a battery can be prevented so that a battery
configuration not easily allowing a performance reduction as caused
by temperature increase is yielded. Furthermore, by having a heat
resistant insulating layer, mechanical strength of a separator
having a heat resistant insulating layer is improved so that the
separator hardly has a film breaking. Moreover, because of the
effect of inhibiting thermal shrinkage and a high level of
mechanical strength, the separator is hardly curled during the
process of producing a battery.
[0141] The inorganic particles in a heat resistant insulating layer
contribute to the mechanical strength or the effect of inhibiting
thermal shrinkage of the heat resistant insulating layer. The
material used as inorganic particles is not particularly limited.
Examples thereof include oxides (SiO.sub.2, Al.sub.2O.sub.3,
ZrO.sub.2, TiO.sub.2), hydroxides, and nitrides of silicon,
aluminum, zirconium, and titanium, and a composite thereof. These
inorganic particles may be derived from mineral resources such as
boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and
mica, or artificially synthesized. Furthermore, these inorganic
particles may be used alone or in combination of two or more kinds
thereof. Among them, from the viewpoint of the cost, it is
preferable to use silica (SiO.sub.2) or alumina (Al.sub.2O.sub.3),
and it is more preferable to use alumina (Al.sub.2O.sub.3).
[0142] The weight per unit area of heat resistant particles is not
particularly limited, but is preferably 5 to 15 g/m.sup.2. When the
weight per unit area is within this range, sufficient ion
conductivity is obtained and heat resistant strength is maintained,
which is preferable.
[0143] The binder in a heat resistant insulating layer has a role
of attaching inorganic particles to each other or attaching
inorganic particles to a porous resin substrate layer. With this
binder, the heat resistant insulating layer is stably formed and
peeling between a porous substrate layer and a heat resistant
insulating layer is prevented.
[0144] The binder used for a heat resistant insulating layer is not
particularly limited, and examples thereof which can be used as the
binder include compounds such as carboxymethyl cellulose (CMC),
polyacrylonitrile, cellulose, an ethylene-vinyl acetate copolymer,
polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene
rubber, butadiene rubber, polyvinylidene fluoride (PVDF),
polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), and
methyl acrylate. Among these, carboxymethyl cellulose (CMC), methyl
acrylate, or polyvinylidene fluoride (PVDF) is preferably used.
These compounds may be used alone or in combination of two or more
kinds thereof.
[0145] The content of the binder in the heat resistant insulating
layer is preferably 2 to 20% by weight with respect to 100% by
weight of the heat resistant insulating layer. When the content of
the binder is 2% by weight or more, the peeling strength between
the heat resistant insulating layer and a porous substrate layer
can be increased and vibration resistance of a separator can be
enhanced. On the other hand, when the content of the binder is 20%
by weight or less, a gap between inorganic particles is maintained
at an appropriate level so that sufficient lithium ion conductivity
can be ensured.
[0146] The thermal shrinkage rates of a separator having a heat
resistant insulating layer for both MD and TD are 10% or less after
maintaining for 1 hour at conditions of 150.degree. C. and 2
gf/cm.sup.2. By using a material with such high heat resistance,
shrinkage of a separator can be effectively prevented even when the
internal temperature of a battery reaches 150.degree. C. due to
increased heat generation amount from a positive electrode. As a
result, an occurrence of a short between electrodes of a battery
can be prevented, and thus a battery configuration not easily
allowing performance reduction due to temperature increase is
yielded.
[0147] <<Positive Electrode Current Collecting Plate and
Negative Electrode Current Collecting Plate>>: The material
for forming a current collecting plate (25, 27) is not particularly
limited, and a known highly conductive material which has been
conventionally used for a current collecting plate for a lithium
ion secondary battery can be used. Preferred examples of the
material for forming a current collecting plate include metal
materials such as aluminum, copper, titanium, nickel, stainless
steel (SUS), and an alloy thereof. From the viewpoint of light
weightiness, resistance to corrosion, and high conductivity,
aluminum and copper are more preferable. Aluminum is particularly
preferable. Incidentally, the same material or a different material
may be used for the positive electrode current collecting plate 25
and negative electrode current collecting plate 27.
[0148] <<Positive Electrode Lead and Negative Electrode
Lead>>: Further, although it is not illustrated, the current
collector 11 and the current collecting plate (25, 27) may be
electrically connected to each other via a positive electrode lead
or a negative electrode lead. The same material used for a lithium
ion secondary battery of a related art can be also used as a
material for forming the positive and negative electrode leads.
Incidentally, a portion led from an outer casing is preferably
coated with a heat resistant and insulating thermally shrunken tube
or the like so that it has no influence on a product (for example,
an automobile component, in particular, an electronic device or the
like) according to electric leak after contact with peripheral
devices or wirings.
[0149] <<Sealing Portion>>: The sealing portion
(insulation layer) has a function of preventing contact between the
current collectors adjacent to each other and preventing a short
circuit caused at the end portion of the single battery layer. The
material constituting the sealing portion may be any materials as
long as it has an insulation property, a sealing property (sealing
performance) to prevent the solid electrolyte from coming off and
prevent permeation of external moisture, heat resistance under
battery operation temperature and the like. Examples of the
material include an acrylic resin, a urethane resin, an epoxy
resin, a polyethylene resin, a polypropylene resin, a polyimide
resin, and rubber (ethylene-propylene-diene rubber: EPDM).
Alternatively, an isocyanate adhesive, an acrylic resin adhesive, a
cyanoacrylate adhesive, or the like may be used, and a hot-melt
adhesive (urethane resin, polyamide resin, polyolefin resin) may
also be used. Among these, from the viewpoint of corrosion
resistance, chemical resistance, ease of production (film-forming
performance), economical efficiency, and the like, a polyethylene
resin or a polypropylene resin is preferably used as a constituent
material of the insulation layer, and a resin containing an
amorphous polypropylene resin as a main component and obtained by
copolymerizing ethylene, propylene, and butene is preferably
used.
[0150] <<Battery Outer Casing Body>>: As a battery
outer casing body, an envelope-shaped casing capable of covering a
power generating element as illustrated in FIG. 1, in which a
laminate film 29 including aluminum is contained, may be used in
addition to a known metal can casing. As for the laminate film, a
laminate film with a three-layered structure formed by laminating
PP, aluminum, and nylon in this order can be used, but is not
limited thereto. From the viewpoint of having higher output power
and excellent cooling performance, and of being suitably usable for
a battery for a large instrument such as an EV or an HEV, a
laminate film is desirable. In addition, since the group pressure
applied from outside to a power generating element can be easily
controlled and thus the thickness of an electrolyte solution layer
can be easily controlled to a desired value, an aluminate laminate
is more preferred for an outer casing body.
[0151] In the bipolar type secondary battery of this embodiment,
when a positive electrode active material layer or a negative
electrode active material layer is configured by using the
above-described sheet-shaped electrode, the stress caused by
expansion and shrinkage of an active material is alleviated even
when an active material having a large battery capacity is used,
and thus the cycle characteristics of the battery can be improved.
Therefore, the bipolar type secondary battery of this embodiment is
suitably used as a power source for operating an EV or an HEV.
[0152] <<Cell Size>>: FIG. 3 is a perspective view
illustrating the appearance of a flat lithium ion secondary battery
as a representative embodiment of a secondary battery.
[0153] As illustrated in FIG. 3, a flat lithium ion secondary
battery 50 has a flat and rectangular shape, and from both sides, a
positive electrode tab 58 and a negative electrode tab 59 are drawn
to extract electric power. A power generating element 57 is covered
by a battery outer casing material (laminate film 52) of the
lithium ion secondary battery 50 with its periphery fused by heat.
The power generating element 57 is sealed in a state in which the
positive electrode tab 58 and the negative electrode tab 59 are led
to the outside. Herein, the power generating element 57 corresponds
to the power generating element 21 of the bipolar type secondary
battery 10 illustrated in FIG. 2 as described above. In the power
generating element 57, plural single battery layers (single cell)
19, which are each formed of the positive electrode (positive
electrode active material layer) 15, the electrolyte layer 17, and
the negative electrode (negative electrode active material layer)
13, are laminated.
[0154] Incidentally, the lithium ion secondary battery is not
limited to a flat shape of laminate type. The winding type lithium
ion secondary battery may have a barrel shape or a flat and
rectangular shape obtained by modifying the barrel shape, and there
is no particular limitation. As an outer casing material of the
barrel shape, a laminate film may be used, or a barrel can (metal
can) of a related art may be used, and thus there is no particular
limitation. Preferably, the power generating element is encased
with an aluminum laminate film. The weight reduction may be
achieved with such shape.
[0155] Further, drawing of the tabs 58 and 59 illustrated in FIG. 3
is also not particularly limited. The positive electrode tab 58 and
the negative electrode tab 59 may be drawn from the same side or
each of the positive electrode tab 58 and the negative electrode
tab 59 may be divided into plural tabs and drawn from each side,
thus there is no particular limitation on the embodiment
illustrated in FIG. 3. In addition, in a winding type lithium ion
battery, it is also possible to form a terminal by using, for
example, a barrel can (metal can) instead of a tab.
[0156] A typical electric vehicle has a battery storage space of
about 170 L. Since a cell and an auxiliary machine such as a device
for controlling charge and discharge are stored in this space,
storage space efficiency of a cell is generally about 50%. The cell
loading efficiency for this space is a factor of determining the
cruising distance of an electric vehicle. As the size of a single
cell decreases, the loading efficiency is lowered, and thus it
becomes impossible to maintain the cruising distance.
[0157] Therefore, in the present invention, the battery structure
of which power generating element is covered with an outer casing
body preferably has a large size. Specifically, the length of the
short side of a laminate cell battery is preferably 100 mm or more.
Such a large-sized battery can be used for an automobile. Herein,
the length of the short side of the laminate cell battery indicates
the length of the shortest side. The upper limit of the length of
the short side is not particularly limited, but is generally 400 mm
or less.
[0158] <<Volume Energy Density and Rated Discharge
Capacity>>: According to the market requirement, a typical
electric vehicle needs to have driving distance (cruising distance)
of 100 km per single charge. Considering such a cruising distance,
the volume energy density of a battery is preferably 157 Wh/L or
more, and the rated capacity is preferably 20 Wh or more.
[0159] Further, it is also possible to define the large size of a
battery in view of a relation of battery area or battery capacity,
from the viewpoint of a large-sized battery, which is different
from a physical size of an electrode. For example, in the case of a
flat and stack type laminate battery, the problem of having lowered
battery characteristics (cycle characteristics), which is caused by
the collapse of the crystal structure and the like accompanying
expansion and shrinkage of an active material, may occur more
easily in a battery having a value of a ratio of the battery area
(projected area of a battery including a battery outer casing body)
to the rated capacity is 5 cm.sup.2/Ah or more and having a rated
capacity of 3 Ah or more since the battery area per unit capacity
is large. Therefore, the non-aqueous electrolyte secondary battery
according to this embodiment is preferably a large-sized battery as
described above from the viewpoint of having a larger merit
obtained from exhibition of the working effects of the present
invention. Furthermore, an aspect ratio of a rectangular electrode
is preferably 1 to 3, and more preferably 1 to 2. Incidentally, the
aspect ratio of the electrode is defined by the
longitudinal/transversal ratio of a positive electrode active
material layer with a rectangular shape. When the aspect ratio is
set to be in such a range, an advantage of having both performances
required for a vehicle and loading space can be obtained.
[0160] <<Assembled Battery>>: An assembled battery is
formed by connecting plural batteries. Specifically, at least two
of them are used in series, in parallel, or in series and parallel.
According to arrangement in series or parallel, it is possible to
freely adjust the capacity and voltage.
[0161] It is also possible to form a detachable small-size
assembled battery by connecting plural batteries in series or in
parallel. Further, by connecting again plural detachable small-size
assembled batteries in series or parallel, an assembled battery
having high capacity and high output, which is suitable for a power
source or an auxiliary power source for operating a vehicle
requiring a high volume energy density and a high volume output
density, can be formed. The number of the connected batteries for
producing an assembled battery or the number of the stacks of a
small-size assembled battery for producing an assembled battery
with high capacity may be determined depending on the capacity or
output of a battery of a vehicle (electric vehicle) on which the
battery is mounted.
[0162] <<Vehicle>>: The non-aqueous electrolyte
secondary battery of the present invention can maintain discharge
capacity even when it is used for a long period of time, and thus
has good cycle characteristics. Further, the non-aqueous
electrolyte secondary battery has a high volume energy density. For
use in a vehicle such as an electric vehicle, a hybrid electric
vehicle, a fuel cell electric vehicle, or a hybrid fuel cell
electric vehicle, a long service life is required as well as high
capacity and large size compared to use for an electric and mobile
electronic device. Therefore, the non-aqueous electrolyte secondary
battery described above can be preferably used as a power source
for a vehicle, for example, as a power source for operating a
vehicle or as an auxiliary power source for operating a
vehicle.
[0163] Specifically, the battery or an assembled battery formed by
combining plural batteries can be mounted on a vehicle. According
to the present invention, a battery with excellent long term
reliability and output characteristics, and a long service life can
be formed, and thus, by mounting this battery, a plug-in hybrid
electric vehicle with a long EV driving distance and an electric
vehicle with a long driving distance per charge can be achieved.
This is because, when the battery or an assembled battery formed by
combining plural batteries is used for, for example, a vehicle such
as a hybrid car, a fuel cell electric car, and an electric car
(including a two-wheel vehicle (motor bike) or a three-wheel
vehicle in addition to all four-wheel vehicles (an automobile, a
truck, a commercial vehicle such as a bus, a compact car, or the
like)), a vehicle with a long service life and high reliability can
be provided. However, the use is not limited to a vehicle, and it
can be applied to various power sources of other transportation
means, for example, a moving object such as an electric train, and
it can be also used as a power source for loading such as an
uninterruptable power source device.
[0164] Hereinafter, the description is made below in more detail by
means of Examples and Comparative Examples, but the present
invention is not limited only to the Examples described below.
Production Example 1
[0165] Synthesis of Gel-Forming Polymer (Polyethylene Glycol-Based
Polyurethane Resin). To a four-necked flask equipped with a stirrer
and a thermometer, 57.4 parts by mass of polyethylene glycol having
a number average molecular weight (as calculated from an OH value)
of 6,000, 8.0 parts by mass of ethylene glycol, 34.7 parts by mass
of diphenylmethane-4,4'-diisocyanate (MDI), and 233 parts by mass
of N,N-dimethylformamide (DMF) were introduced and reacted at
70.degree. C. for 10 hours under a dried nitrogen atmosphere,
thereby obtaining a polyurethane resin solution having a resin
concentration of 30% by mass and a viscosity of 600 poise (60 Pas,
20.degree. C.).
[0166] The polyurethane resin solution obtained in this way was
cast on a PET film and then dried to form a sheet-shaped film
having a thickness of 500 .mu.m, and then the sheet-shaped film was
punched in a dumbbell form. Then, after the film was immersed in an
electrolyte solution (1M LiPF.sub.6, ethylene carbonate
(EC)/diethyl carbonate (DEC)=3/7 (volume ratio)) at 50.degree. C.
for 3 days, a value of the tensile elongation at break was measured
in accordance with ASTM D683 (test piece shape Type II), and as a
result, was 50%.
Production Example 2
[0167] Carbon Coating of LiCoO.sub.2 Particle (Sintering Method).
Into a beaker, 50 g of distilled water was put and heated to
90.degree. C., and 1.0 g of polyvinyl alcohol as a water-soluble
polymer was added thereto and was dissolved under stirring. After
10.0 g of LiCoO.sub.2 powder as an active material was added to a
water-soluble polymer aqueous solution thus prepared so as to
disperse the active material, moisture was evaporated by heating
and stirring. After the obtained solid was further dried at
120.degree. C., the dried solid was put into an electric furnace at
400.degree. C. so as to be heated for 30 minutes, and then was
taken out of the furnace and allowed to cool. It was confirmed by
the SEM observation that the surface of LiCoO.sub.2 particle was
carbon-coated.
[0168] Coating of Active Material with Gel-Forming Polymer
(Polyethylene Glycol-Based Polyurethane Resin) Containing Acetylene
Black (Poor Solvent Precipitation Method). To a 1 L Erlenmeyer
flask, 1.36 parts by mass of the polyurethane resin solution
obtained in Production Example 1 described above (diluted with DMF
to be 19.7% by mass of the resin content), 50 parts by mass of DMF,
and 50 parts by mass of isopropanol (IPA) were introduced and
uniformly stirred. Then, 15 parts by mass of the active material
produced by the above-described method (carbon-coated LiCoO.sub.2
particle) was added thereto, and the resultant mixture was further
stirred for 5 minutes. 100 parts by mass of IPA was further added
dropwise thereto with a dropping funnel.
[0169] After completion of dropping, a solution obtained by
dispersing 0.27 part by mass of acetylene black as a conductive
material in 40 parts by mass of IPA was added, and the resultant
mixture was stirred for 10 minutes. This dispersion liquid was
subjected to filtration under reduced pressure, and thus powder was
filtered.
[0170] The aforementioned operation from the operation in which 50
parts by mass of DMF and 50 parts by mass of IPA were introduced to
the filtered powder was repeated three times in total, thereby
obtaining a core-shell-type electrode material (positive electrode
material) A1 having a core part formed by carbon-coated LiCoO.sub.2
particle and a shell part formed by a gel-forming polymer
(polyethylene glycol-based polyurethane resin) containing acetylene
black. A scanning electron microscope (SEM) photograph
(magnification of 5000) of the core-shell-type electrode material
(positive electrode material) A1 obtained in this way is shown in
FIG. 4.
Production Example 3
[0171] Carbon Coating of LiFePO.sub.4 Particle (Sintering Method).
An active material coated with a carbon-based material was prepared
in the same manner as in Production Example 2 described above,
except that LiCoO.sub.2 of Production Example 2 described above was
changed to LiFePO.sub.4. It was confirmed by the SEM observation
that the surface of LiFePO.sub.4 particle was carbon-coated.
[0172] Coating of Active Material with Gel-Forming Polymer
Containing Acetylene Black (Poor Solvent Precipitation Method). A
core-shell-type electrode material A2 was prepared in the same
manner as in Production Example 2 described above, except that the
active material of Production Example 2 described above
(carbon-coated LiCoO.sub.2 particle) was changed to the active
material prepared by the above-described method (carbon-coated
LiFePO.sub.4 particle). The scanning electron microscope (SEM)
photograph (magnification of 5000) of the obtained core-shell-type
electrode material (positive electrode material) A2 is shown in
FIG. 5.
Production Example 4
[0173] Carbon Coating of Li.sub.4Ti.sub.5O.sub.12 Particle
(Sintering Method). An active material coated with a carbon-based
material was prepared in the same manner as in Production Example 2
described above, except that LiCoO.sub.2 of Production Example 2
described above was changed to Li.sub.4Ti.sub.5O.sub.12. It was
confirmed by the SEM observation that the surface of
Li.sub.4Ti.sub.5O.sub.12 particle was carbon-coated.
[0174] Coating of Active Material with Gel-Forming Polymer
Containing Acetylene Black (Poor Solvent Precipitation Method). A
core-shell-type electrode material A3 was prepared in the same
manner as in Production Example 2 described above, except that the
active material of Production Example 2 described above
(carbon-coated LiCoO.sub.2 particle) was changed to the active
material prepared by the above-described method (carbon-coated
Li.sub.4Ti.sub.5O.sub.12 particle). The scanning electron
microscope (SEM) photograph (magnification of 5000) of the obtained
core-shell-type electrode material (positive electrode material) A3
is shown in FIG. 6.
Production Example 5
[0175] Carbon Coating of LiCoO.sub.2 Particle (Mechanochemical
Method). To 92.6 g of LiCoO.sub.2, 2.4 g of acetylene black (AB)
was added and then treated by a hybridizer so that AB was attached
to the surface of LiCoO.sub.2 and the active material was coated.
At this time, the treatment was continuously carried out for 3
minutes under the treatment conditions of a rotation speed of
15,000 rpm and a load power of 600 W, using a hybridization system
manufactured by NARA MACHINERY CO., LTD. It was confirmed by the
SEM observation that the surface of LiCoO.sub.2 particle was
carbon-coated.
[0176] Coating of Active Material with Gel-Forming Polymer
Containing Acetylene Black (Poor Solvent Precipitation Method). A
core-shell-type electrode material A4 was prepared in the same
manner as in Production Example 2 described above, except that the
active material of Production Example 2 described above
(carbon-coated LiCoO.sub.2 particle by the sintering method) was
changed to the active material prepared by the above-described
method (carbon-coated LiCoO.sub.2 particle by the mechanochemical
method). The scanning electron microscope (SEM) photograph
(magnification of 5000) of the obtained core-shell-type electrode
material (positive electrode material) A4 is shown in FIG. 7.
Comparative Production Example 1
[0177] A core-shell-type electrode material C-A1 was prepared in
the same manner as in Production Example 2 described above, except
that the active material of Production Example 2 described above
(carbon-coated LiCoO.sub.2 particle by the sintering method) was
changed to LiCoO.sub.2 particle which was not carbon-coated.
However, when the obtained core-shell-type electrode material
(positive electrode material) C-A1 was observed by a scanning
electron microscope (SEM) photograph (magnification of 5000), it
was not confirmed that the surface of the active material was
coated with the gel-forming polymer.
Example 1
[0178] Preparation of Test Electrode (Positive Electrode). 85 parts
by mass of the electrode material (positive electrode material) A1
obtained in Production Example 2 described above, 10 parts by mass
of acetylene black as a conductive aid, and 5 parts by mass of
carboxymethyl cellulose (CMC) as a binder were mixed with one
another. Then, water as a solvent for controlling slurry viscosity
was added thereto in an appropriate amount, and then was mixed with
a stirrer, thereby obtaining a positive electrode active material
slurry.
[0179] Meanwhile, an aluminum foil (thickness: 20 .mu.m) was
prepared as a positive electrode current collector. Further, the
positive electrode active material slurry prepared above was
applied to one surface of the positive electrode current collector
such that an amount of the active material applied became 10
mg/cm.sup.2 to thereby form a coating film. Then, this coating film
was dried at 80.degree. C. for 60 minutes and then dried under
vacuum for 6 hours. Thereafter, the obtained positive electrode was
punched using a punch to have a circle shape of .phi.16 mm, thereby
obtaining a test electrode (positive electrode) B1.
Example 2
[0180] In Example 1 described above, the electrode material
(positive electrode material) A1 was changed to 86 parts by mass of
the electrode material (positive electrode material) A2 obtained in
Production Example 3 described above, and the binder was changed to
2 parts by mass of carboxymethyl cellulose (CMC) and 2 parts by
mass of styrene-butadiene rubber (SBR). Except the above changes, a
test electrode (positive electrode) B2 was obtained in the same
manner as in Example 1 described above.
Example 3
[0181] A test electrode (positive electrode) B3 was obtained in the
same manner as in Example 2 described above, except that, in
Example 2 described above, the electrode material (positive
electrode material) A2 was changed to the electrode material
(positive electrode material) A3 obtained in Production Example 4
described above.
Example 4
[0182] A test electrode (positive electrode) B4 was obtained in the
same manner as in Example 1 described above, except that, in
Example 1 described above, the electrode material (positive
electrode material) A1 was changed to the electrode material
(positive electrode material) A4 obtained in Production Example 5
described above.
Comparative Example 1
[0183] 85 parts by mass of carbon-coated LiCoO.sub.2 as a positive
electrode active material (provided that, it was prepared in
Production Example 2 and was not coated with a gel-forming
polymer), 10 parts by mass of acetylene black as a conductive aid,
and 5 parts by mass of carboxymethyl cellulose (CMC) as a binder
were mixed with one another, thereby obtaining a positive electrode
active material slurry.
[0184] Meanwhile, an aluminum foil (thickness: 20 .mu.m) was
prepared as a positive electrode current collector. Further, the
positive electrode active material slurry prepared above was
applied to one surface of the positive electrode current collector
such that an amount of the active material applied became 10
mg/cm.sup.2 to thereby form a coating film. Then, this coating film
was dried at 80.degree. C. for 60 minutes and then dried under
vacuum for 6 hours. Thereafter, the obtained positive electrode was
punched using a punch to have a circle shape of .phi.16 mm, thereby
obtaining a test electrode (positive electrode) C-B1.
Comparative Example 2
[0185] A test electrode (positive electrode) C-B2 was prepared in
the same manner as in Example 2 described above, except that, in
Example 2 described above, the electrode material (positive
electrode material) A2 was changed to an electrode material which
had not been carbon-coated in Production Example 3 described above
(that is, non-treated LiFePO.sub.4 particle).
Comparative Example 3
[0186] A test electrode (positive electrode) C-B3 was prepared in
the same manner as in Example 3 described above, except that, in
Example 3 described above, the electrode material (positive
electrode material) A3 was changed to an electrode material which
had not been carbon-coated in Production Example 4 described above
(that is, non-treated Li.sub.4Ti.sub.5O.sub.12 particle).
[0187] <<Evaluation of Internal Resistance of Test Electrode
(Negative Electrode)>>: Preparation of Coin-Sized Battery.
First, a lithium metal foil (thickness: 500 .mu.m, .phi.17 mm) as a
counter electrode was disposed in an HS cell made of stainless
steel (manufactured by Hohsen Corp.), a separator (thickness: 25
.mu.m, .theta.18 mm) formed by a polypropylene microporous membrane
was placed thereon, and any one of the test electrodes (negative
electrodes) produced in the Examples and Comparative Examples
described above was placed thereon. To a power generating element
obtained in this way, 200 .mu.L of a liquid electrolyte of 1M
LiPF.sub.6/EC+DEC (1:1 (volume ratio)) was injected, a spacer, a
spring, and an upper cover were superimposed in this order, and the
upper cover was fixed with a wing nut, thereby obtaining a
coin-sized battery corresponding to each of the Examples and
Comparative Examples.
[0188] <<Evaluation of Internal Resistance by Alternating
Current Impedance Measurement>>: The coin-sized batteries
produced as described above were charged for 4 hours in total under
the condition of constant current-constant voltage (CC-CV) such
that the upper limit voltage was set to 4.2 V, the lower limit
voltage was set to 3.0 V and the current was set to 0.5 C as for
the LiCoO.sub.2-based electrode. Further, as for the
LiFePO.sub.4-based electrode, the upper limit voltage was set to
4.0 V and the lower limit voltage was set to 3.0 V. Next, after the
constant current discharge was performed to the lower limit voltage
at 0.5 C, the battery was charged to have 50% of the charging
state, and then the alternating current impedance of the cell was
measured from 20 kHz to 0.1 Hz. Regarding a real number component
at 0.3 Hz immediately before the influence of diffusion acted (the
real number component corresponds to the internal resistance of the
battery), the values thereof were read. The ratio of the obtained
values was obtained and evaluated as a ratio of the value of the
internal resistance of the cell of Example to the value of the
internal resistance of the corresponding comparative cell. The
results thereof are presented together in Table 1. Incidentally,
regarding the Li.sub.4Ti.sub.5O.sub.12 electrode battery, in a
battery in which a Li counter electrode was used as a positive
electrode, since the initial state was the charging state, the same
measurement was performed in such a manner that after the lower
limit voltage was set to 1.0 V, the upper limit voltage was set to
2.0 V, and discharge was performed under CC-CV in an initial
period, charge was then performed until 2.0 V, and discharge was
performed to have 50% of the charging state.
TABLE-US-00001 TABLE 1 Ratio of internal resistance of Difference
between cell and comparative cell Electrode cell to Carbon coating
Gel-forming polymer Comparative active comparative Comparative
Comparative Cell cell material cell Example Example Example Example
Example 1 Comparative LiCoO.sub.2 0.61 Present Present Present
Absent Example 1 (sintering method) (sintering method) Example 2
Comparative LiFePO.sub.4 0.81 Present Absent Present Absent Example
2 (sintering method) Example 3 Comparative Li.sub.4Ti.sub.5O.sub.12
0.75 Present Absent Present Absent Example 3 (sintering method)
Example 4 Comparative LiCoO.sub.2 0.86 Present Present Present
Absent Example 1 (mechanochemical (sintering method) method)
[0189] As understood from the result of Table 1, it is found that,
according to the embodiment of the present invention, the internal
resistance can be lowered as compared to the corresponding
Comparative Example.
[0190] In this connection, when the electrode material and the
agent for inhibiting an increase in internal resistance according
to the present invention are used in a non-aqueous electrolyte
secondary battery, it is possible to lower the internal resistance
of the battery, and further, it is possible to lead the way to
provide a battery with excellent rate characteristics and a high
output density. As such, it can be said that the invention of the
present application by which a battery with a high output density
can be provided based on the technical idea considerably different
from the related art has extremely high superiority and is
creative.
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